Prioritization of transactions based on execution progress

ABSTRACT

Controlling access to at least one memory location by a transaction is provided in a multi-processor transactional execution environment. Included is: tracking execution progress of a transaction, the execution progress being a metric of work performed for the transaction which includes at least one of instructions processed or cycles elapsed; based on encountering a conflict with another process for a memory location, comparing execution process of the transaction and execution progress of the other process; and deciding whether to continue the transaction based on the comparing. For instance, based on the execution progress of the transaction being greater than the execution progress of the other process, the transaction is continued, and based on the execution progress of the transaction being less that the execution progress of the other process, then the transaction is aborted.

BACKGROUND

One or more aspects relate, in general, to multiprocessing computing environments, and in particular, to transactional processing within such computing environments.

The number of central processing unit (CPU) cores on a chip and the number of CPU cores connected to a shared memory continues to grow significantly to support growing workload capacity demand. The increasing number of CPUs cooperating to process the same workloads puts a significant burden on software scalability. For example, shared queues or data structures protected by traditional semaphores become hot spots and lead to sub-linear, n-way scaling curves. Traditionally, this has been countered by implementing finer-grain locking and software, and with low-latency/higher-bandwidth interconnects and hardware. Implementing fine-grain locking to improve software scalability can be very complicated and error-prone, and at today's CPU frequencies, the latencies of hardware interconnects are limited by the physical dimension of the chips and systems, and by the speed of light.

Implementations of hardware transactional memory (TM) have been introduced, wherein a group of instructions, called a transaction, operate atomically, and in isolation (sometimes called “serializeability”) on a data structure in memory. The transaction executes optimistically without obtaining a lock, but may need to abort and retry the transaction execution if an operation, of the executing transaction, on a memory location conflicts with another operation on the same memory location. A conflict may occur, for instance, when one processor core writes data that another processor core is reading. A transactional core conventionally does not have any means to avoid or defer a conflict, and thus, it aborts the transaction.

SUMMARY

Shortcomings of the prior art are overcome and additional advantages are provided through the provision of a method of accessing at least one memory location by one of a plurality of transactions in a multi-processor transactional execution environment, the method including: tracking execution progress of a transaction, the execution progress being a metric of work performed for the transaction and comprising at least one of transaction instructions processed or cycles elapsed; based upon encountering a conflict with another process for a memory location, comparing the execution progress of the transaction and the execution progress of the other the other process; and deciding whether to continue the transaction based on the comparing of the execution progress of the transaction and the execution progress of the other the other process.

Advantageously, the method(s) described herein of accessing at least one memory location by one of a plurality of transactions in a multi-processor transactional execution environment enhances operation of the computing environment. For instance, execution progress of a transaction is tracked, and based on encountering a conflict with another process for a memory location, execution progress of the transaction and the other the other process are compared, and a decision is made whether to continue the transaction based on the results of the comparison. Thus, the transaction or process which has performed the greater amount of work is given priority over the transaction which has performed less.

By way of enhancement, in one or more embodiments, the deciding may include: based on the execution progress of the transaction being greater than the execution progress of the other the other process, continuing the transaction; and based on the execution progress of the transaction being less than the execution progress of the other the other process, aborting the transaction.

In one or more embodiments, the other the other process is another transaction. By way of example, the other the other transaction may send the transaction a protocol request packet which includes the execution progress of the other the other transaction.

In one or more implementations, based on the execution progress of the transaction being greater than the execution progress of the other the other transaction, a response is sent to the other the other transaction rejecting access to the memory location; and based on the execution progress of the transaction being less than the execution progress of the other the other transaction, a response is sent to the other the other transaction which includes data corresponding to the memory location.

In one or more other embodiments, based on encountering the conflict, and prior to performing the comparing, the method includes: determining whether the transaction or the other the other transaction is a super-transaction; and based on one of the transaction or the other the other transaction being the super-transaction, continuing the super-transaction, and refraining from the comparing.

In one or more other implementations, the method includes, based on encountering the conflict, and prior to performing the comparing: determining whether the transaction or the other the other transaction has a higher assigned conflict priority; and based on one of the transaction or the other the other transaction having a higher assigned conflict priority, continuing that transaction and refraining from the comparing.

In one or more embodiments, a metric of work performed for the transaction is incremented based on transaction instructions processed or cycles elapsed in a decode stream for the transaction, and the method includes, based on encountering the conflict, and prior to performing the comparing, determining whether the transaction or the other the other transaction has a higher assigned priority designation, and based on one of the transaction or the other the other transaction having the higher assigned priority designation, continuing that transaction and refraining from the comparing.

In still another embodiment, the metric of work performed may include at least one of transaction instructions executed since a TBEGIN instruction, or cycles elapsed since the TBEGIN instruction.

System and computer program products relating to one or more of the aspects noted herein are also described and claimed.

Additional features and advantages are realized through the techniques described herein. Other embodiments and aspects are described in detail herein, and are considered part of the claimed aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more aspects are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 depicts one embodiment of a computing environment;

FIG. 2A depicts one example of a Transaction Begin (TBEGIN) instruction;

FIG. 2B depicts one embodiment of further details of a field of the TBEGIN instruction of FIG. 2A;

FIG. 3A depicts on example of a Transaction Begin Constrained (TBEGINC) instruction;

FIG. 3B depicts one embodiment of further details of a field of the TBEGINC instruction of FIG. 3A;

FIG. 4 depicts one example of a Transaction End (TEND) instruction;

FIG. 5 depicts one example of a Transaction Abort (TABORT) instruction;

FIG. 6 depicts one example of nested transactions;

FIG. 7 depicts one example of a transaction diagnostic block;

FIGS. 8A-8C depict exemplary embodiments of logic for controlling access to a memory location within a multi-processor transactional execution environment, in accordance with one or more aspects of the present invention;

FIGS. 9A & 9B depict another exemplary embodiment of logic for controlling access to a memory location in a multi-processor transactional execution environment, in accordance with one or more aspects of the present invention;

FIG. 10A depicts another example of a computing environment to incorporate and/or use one or more aspects of the present invention;

FIG. 10B depicts further details of the memory of FIG. 10A;

FIG. 11 depicts one embodiment of a host computer system;

FIG. 12 depicts one embodiment of various elements of a computer system;

FIG. 13A depicts one embodiment of the execution unit of the computer system of FIG. 12;

FIG. 13B depicts one embodiment of the branch unit of the computer system of FIG. 12;

FIG. 13C depicts one embodiment of the load/store unit of the computer system of FIG. 12;

FIG. 14 depicts one embodiment of a cloud computing node;

FIG. 15 depicts one embodiment of a cloud computing environment; and

FIG. 16 depicts one example of abstraction model layers.

DETAILED DESCRIPTION

The accompanying figures, in which like reference numerals refer to identical or functionally similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention. As understood by one of skill in the art, the accompanying figures are provided for ease of understanding and illustrate aspects of certain embodiments of the present invention. The invention is not limited to the embodiments depicted in the figures.

As understood by one of skill in the art, program code, as referred to throughout this application, may include both software and hardware. For example, program code in certain embodiments of the present invention includes fixed function hardware, while other embodiments utilize a software-based implementation of the functionality described. Certain embodiments combine both types of program code.

One or more aspects of the present invention provided herein relate to a multi-processor transactional execution environment, and more particularly, to controlling access to a memory location when a conflict is encountered between, for instance, a transaction and another process, such as another transaction. The access facility disclosed herein utilizes execution progress of the transaction, as well as execution progress of the other the other process, in deciding whether to continue the transaction. The execution progress is, in one or more implementations, a metric of work performed for the transaction or process, and in one or more implementations, based on the execution progress of the transaction being greater than the execution progress of the other the other process, the transaction continues, and based on it being less, the transaction is aborted. This advantageously facilitates processing within the computing environment (e.g., multi-processor transaction execution environment), by allowing the transaction or process that has performed the greater amount of work to continue, while the other access request is denied.

Within a multi-processor transactional execution environment, methods, systems, and computer program products are provided for controlling access to at least one memory location by one of a plurality of transactions. As explained further herein, when, for instance, two transactions are in interference for a memory location, one transaction needs to be aborted. Pursuant to the concepts disclosed herein, in one or more implementations, it is desirable to abort the transaction that has performed less work, as opposed to the transaction that may have undergone significant processing. In accordance with the concepts disclosed, execution progress of a transaction is tracked. The metric used to track the execution progress is representative of the work performed for the transaction, and may include at least one of transaction instructions processed or cycles elapsed since, for instance, beginning execution of the transaction.

The access approach may be further enhanced by taking into account one or more static, associated priorities. For instance, a super-transaction may be defined as a transaction having priority over other transactions of the plurality of transactions. Further, one or more statically-defined conflict priorities may be assigned to the transactions. In such cases, the statically-defined priorities may be checked in deciding whether one transaction has priority over another. If not, then the amount of work performed by the respective transactions may be compared in deciding whether to continue one transaction over another, as explained further below with reference to FIGS. 8A-9.

Prior to describing exemplary embodiments of the access logic disclosed herein, details regarding the transactional execution facility, including non-constrained and constrained transactional execution modes, are discussed below with reference to FIGS. 1-7.

The transactional execution facility introduces a CPU state called the transactional execution (TX) mode. Following a CPU reset, the CPU is not in the TX mode. The CPU enters the TX mode by a TRANSACTION BEGIN instruction, and leaves the TX mode by either (a) an outermost TRANSACTION END instruction (more details on inner and outer to follow), (b) a CONDITIONAL TRANSACTION END instruction that sets the condition code to 0; or (c) the transaction being aborted. While in the TX mode, storage accesses by the CPU appear to be block-concurrent as observed by other CPUs and the I/O subsystem. The storage accesses are either (a) committed to storage when the outermost transaction ends without aborting (i.e., e.g., updates made in a cache or buffer local to the CPU are propagated and stored in real memory and visible to other CPUs), or (b) discarded if the transaction is aborted.

Transactions may be nested. That is, while the CPU is in the TX mode, it may execute another TRANSACTION BEGIN instruction. The instruction that causes the CPU to enter the TX mode is called the outermost TRANSACTION BEGIN; similarly, the program is said to be in the outermost transaction. Subsequent executions of TRANSACTION BEGIN are called inner instructions; and the program is executing an inner transaction. The model provides a minimum nesting depth and a model-dependent maximum nesting depth. An EXTRACT TRANSACTION NESTING DEPTH instruction returns the current nesting depth value, and in a further embodiment, may return a maximum nesting-depth value. This technique uses a model called “flattened nesting” in which an aborting condition at any nesting depth causes all levels of the transaction to be aborted, and control is returned to the instruction following the outermost TRANSACTION BEGIN.

During processing of a transaction, a transactional access made by one CPU is said to conflict with either (a) a transactional access or non-transactional access made by another CPU, or (b) a non-transactional access made by the I/O subsystem, if both accesses are to any location within the same cache line, and one or both of the accesses is a store. In other words, in order for transactional execution to be productive, the CPU is not to be observed making transactional accesses until it commits. This programming model may be highly effective in certain environments; for example, the updating of two points in a doubly-linked list of a million elements. However, it may be less effective, if there is a lot of contention for the storage locations that are being transactionally accessed.

In one model of transactional execution (referred to herein as a non-constrained transaction), when a transaction is aborted, the program may either attempt to re-drive the transaction in the hopes that the aborting condition is no longer present, or the program may “fall back” to an equivalent non-transactional path. In another model of transactional execution (referred to herein as a constrained transaction), an aborted transaction is automatically re-driven by the CPU; in the absence of constraint violations, the constrained transaction is assured of eventual completion.

When initiating a transaction, the program can specify various controls, such as (a) which general registers are restored to their original contents if the transaction is aborted, (b) whether the transaction is allowed to modify the floating-point-register context, including, for instance, floating point registers and the floating point control register, (c) whether the transaction is allowed to modify access registers (ARs), and (d) whether certain program-exception conditions are to be blocked from causing an interruption. If a non-constrained transaction is aborted, various diagnostic information may be provided. For instance, the outermost TBEGIN instruction that initiates a non-constrained transaction may designate a program specified transaction diagnostic block (TDB). Further, the TDB in the CPU's prefix area or designated by the host's state description may also be used if the transaction is aborted due to a program interruption or a condition that causes interpretative execution to end, respectively.

Indicated above are various types of registers. These are further explained in detail herein. General registers may be used as accumulators in general arithmetic and logical operations. In one embodiment, each register contains 64 bit positions, and there are 16 general registers. The general registers are identified by the numbers 0-15, and are designated by a four-bit R field in an instruction. Some instructions provide for addressing multiple general registers by having several R fields. For some instructions, the use of a specific general register is implied rather than explicitly designated by an R field of the instruction.

In addition to their use as accumulators in general arithmetic and logical operations, 15 of the 16 general registers are also used as base address and index registers in address generation. In these cases, the registers are designated by a four-bit B field or X field in an instruction. A value of zero in the B or X field specifies that no base or index is to be applied, and thus, general register 0 is not to be designated as containing a base address or index.

Floating point instructions use a set of floating point registers. The CPU has 16 floating point registers, in one embodiment. The floating point registers are identified by the numbers 0-15, and are designated by a four bit R field in floating point instructions. Each floating point register is 64 bits long and can contain either a short (32-bit) or a long (64-bit) floating point operand.

A floating point control (FPC) register is a 32-bit register that contains mask bits, flag bits, a data exception code, and rounding mode bits, and is used during processing of floating point operations.

Further, in one embodiment, the CPU has 16 control registers, each having 64 bit positions. The bit positions in the registers are assigned to particular facilities in the system, such as Program Event Recording (PER) (discussed below), and are used either to specify that an operation can take place or to furnish special information required by the facility. In one embodiment, for the transactional facility, CR0 (bits 8 and 9) and CR2 (bits 61-63) are used, as described below.

The CPU has, for instance, 16 access registers numbered 0-15. An access register consists of 32 bit positions containing an indirect specification of an address space control element (ASCE). An address space control element is a parameter used by the dynamic address translation (DAT) mechanism to translate references to a corresponding address space. When the CPU is in a mode called the access register mode (controlled by bits in the program status word (PSW)), an instruction B field, used to specify a logical address for a storage operand reference, designates an access register, and the address space control element specified by the access register is used by DAT for the reference being made. For some instructions, an R field is used instead of a B field. Instructions are provided for loading and storing the contents of the access registers and for moving the contents of one access register to another.

Each of access registers 1-15 can designate any address space. Access register 0 designates the primary address space. When one of access registers 1-15 is used to designate an address space, the CPU determines which address space is designated by translating the contents of the access register. When access register 0 is used to designate an address space, the CPU treats the access register as designating the primary address space, and it does not examine the actual contents of the access register. Therefore, the 16 access registers can designate, at any one time, the primary address space and a maximum of 15 other spaces.

In one embodiment, there are multiple types of address spaces. An address space is a consecutive sequence of integer numbers (virtual addresses), together with the specific transformation parameters which allow each number to be associated with a byte location in storage. The sequence starts at zero and proceeds left to right.

In, for instance, the z/Architecture, when a virtual address is used by a CPU to access main storage (a.k.a., main memory), it is first converted, by means of dynamic address translation (DAT), to a real address, and then, by means of prefixing, to an absolute address. DAT may use from one to five levels of tables (page, segment, region third, region second, and region first) as transformation parameters. The designation (origin and length) of the highest-level table for a specific address space is called an address space control element, and it is found for use by DAT in a control register or as specified by an access register. Alternatively, the address space control element for an address space may be a real space designation, which indicates that DAT is to translate the virtual address simply by treating it as a real address and without using any tables.

DAT uses, at different times, the address space control elements in different control registers or specified by the access registers. The choice is determined by the translation mode specified in the current PSW. Four translation modes are available: primary space mode, secondary space mode, access register mode and home space mode. Different address spaces are addressable depending on the translation mode.

At any instant when the CPU is in the primary space mode or secondary space mode, the CPU can translate virtual addresses belonging to two address spaces—the primary address space and the second address space. At any instant when the CPU is in the access register mode, it can translate virtual addresses of up to 16 address spaces—the primary address space and up to 15 AR-specified address spaces. At any instant when the CPU is in the home space mode, it can translate virtual addresses of the home address space.

The primary address space is identified as such because it consists of primary virtual addresses, which are translated by means of the primary address space control element (ASCE). Similarly, the secondary address space consists of secondary virtual addresses translated by means of the secondary ASCE; the AR specified address spaces consist of AR specified virtual addresses translated by means of AR specified ASCEs; and the home address space consists of home virtual addresses translated by means of the home ASCE. The primary and secondary ASCEs are in control registers 1 and 7, respectively. AR specified ASCEs are in ASN-second-table entries that are located through a process called access-register translation (ART) using control registers 2, 5 and 8. The home ASCE is in control register 13.

One embodiment of a computing environment to incorporate and use one or more aspects of the transactional facility described herein is described with reference to FIG. 1.

Referring to FIG. 1, in one example, a computing environment 100 is based on the z/Architecture, offered by International Business Machines (IBM®) Corporation, Armonk, N.Y. The z/Architecture is described in an IBM Publication entitled “z/Architecture Principles of Operation,” IBM Publication No. SA22-7832-10, March 2015, which is hereby incorporated herein by reference in its entirety.

Z/ARCHITECTURE, IBM, and Z/OS and Z/VM (referenced below) are registered trademarks of International Business Machines Corporation, Armonk, N.Y. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies.

As one example, computing environment 100 includes a central processor complex (CPC) 102 coupled to one or more input/output (I/O) devices 106 via one or more control units 108. Central processor complex 102 includes, for instance, a processor memory 104 (a.k.a., main memory, main storage, central storage) coupled to one or more central processors (a.k.a., central processing units (CPUs)) 110, and an input/output subsystem 111, each of which is described below.

Processor memory 104 includes, for example, one or more partitions 112 (e.g., logical partitions), and processor firmware 113, which includes a logical partition hypervisor 114 and other processor firmware 115. One example of logical partition hypervisor 114 is the Processor Resource/System Manager (PRISM), offered by International Business Machines Corporation, Armonk, N.Y.

A logical partition functions as a separate system and has one or more applications 120, and optionally, a resident operating system 122 therein, which may differ for each logical partition. In one embodiment, the operating system is the z/OS operating system, the z/VM operating system, the z/Linux operating system, or the TPF operating system, offered by International Business Machines Corporation, Armonk, N.Y. Logical partitions 112 are managed by logical partition hypervisor 114, which is implemented by firmware running on processors 110. As used herein, firmware includes, e.g., the microcode and/or millicode of the processor. It includes, for instance, the hardware-level instructions and/or data structures used in implementation of higher level machine code. In one embodiment, it includes, for instance, proprietary code that is typically delivered as microcode that includes trusted software or microcode specific to the underlying hardware and controls operating system access to the system hardware.

Central processors 110 are physical processor resources allocated to the logical partitions. In particular, each logical partition 112 has one or more logical processors, each of which represents all or a share of a physical processor 110 allocated to the partition. The logical processors of a particular partition 112 may be either dedicated to the partition, so that the underlying processor resource 110 is reserved for that partition; or shared with another partition, so that the underlying processor resource is potentially available to another partition. In one example, one or more of the CPUs include aspects of the transactional execution facility 130 and conditional transaction end facility 132 described herein.

Input/output subsystem 111 directs the flow of information between input/output devices 106 and main storage 104. It is coupled to the central processing complex, in that it can be a part of the central processing complex or separate therefrom. The I/O subsystem relieves the central processors of the task of communicating directly with the input/output devices and permits data processing to proceed concurrently with input/output processing. To provide communications, the I/O subsystem employs I/O communications adapters. There are various types of communications adapters including, for instance, channels, I/O adapters, PCI cards, Ethernet cards, Small Computer Storage Interface (SCSI) cards, etc. In the particular example described herein, the I/O communications adapters are channels, and therefore, the I/O subsystem is referred to herein as a channel subsystem. However, this is only one example. Other types of I/O subsystems can be used.

The I/O subsystem uses one or more input/output paths as communication links in managing the flow of information to or from input/output devices 106. In this particular example, these paths are called channel paths, since the communication adapters are channels.

The computing environment described above is only one example of a computing environment that can be used. Other environments, including but not limited to, non-partitioned environments, other partitioned environments, and/or emulated environments, may be used; embodiments are not limited to any one environment.

In accordance with one or more aspects, the transactional execution facility is an enhancement of the central processing unit that provides the means by which the CPU can execute a sequence of instructions—known as a transaction—that may access multiple storage locations, including the updating of those locations. As observed by other CPUs and the I/O subsystem, the transaction is either (a) completed in its entirety as a single atomic operation, or (b) aborted, potentially leaving no evidence that it ever executed (except for certain conditions described herein). Thus, a successfully completed transaction can update numerous storage locations without any special locking that is needed in the classic multiprocessing model.

The transactional execution facility includes, for instance, one or more controls; one or more instructions; transactional processing, including constrained and non-constrained execution; and abort processing, each of which is further described below.

In one embodiment, three special purpose controls, including a transaction abort Program Status Word (PSW), a transaction diagnostic block (TDB) address, and a transaction nesting depth; five control register bits; and a plurality of general instructions, including TRANSACTION BEGIN (constrained and non-constrained), TRANSACTION END, EXTRACT TRANSACTION NESTING DEPTH, TRANSACTION ABORT, and NONTRANSACTIONAL STORE are used to control the transactional execution facility. When the facility is installed, it is installed, for instance, in all CPUs in the configuration. A facility indication, bit 73 in one implementation, when one, indicates that the transactional execution facility is installed.

Further, in one aspect, when the transactional execution facility is installed, another facility, referred to as the conditional transaction end facility, may also be installed. The conditional transaction end facility is installed when, for instance, bit 55 of the facility indication bits is set to one. In one implementation, this bit is meaningful only when bit 73 representing the transactional execution facility is also one. When both the facilities are installed, then the CONDITIONAL TRANSACTION END instruction is also an enhancement of the CPU and is used to control transactional execution.

When the transactional execution facility is installed, the configuration provides a non-constrained transactional execution facility, and optionally, a constrained transactional execution facility, each of which is described below. When facility indications 50 and 73, as examples, are both one, the constrained transactional execution facility is installed. Both facility indications are stored in memory at specified locations.

As used herein, the instruction name TRANSACTION BEGIN refers to the instructions having the mnemonics TBEGIN (Transaction Begin for a non-constrained transaction) and TBEGINC (Transaction Begin for a constrained transaction). Discussions pertaining to a specific instruction are indicated by the instruction name followed by the mnemonic in parentheses or brackets, or simply by the mnemonic.

One embodiment of a format of a TRANSACTION BEGIN (TBEGIN) instruction is depicted in FIGS. 2A-2B. As one example, a TBEGIN instruction 200 (FIG. 2A) includes an opcode field 202 that includes an opcode specifying a transaction begin non-constrained operation; a base field (B₁) 204; a displacement field (D₁) 206; and an immediate field (I₂) 208. When the B₁ field is nonzero, the contents of the general register specified by B₁ 204 are added to D₁ 206 to obtain the first operand address.

When the B₁ field is nonzero, the following applies:

-   -   When the transaction nesting depth is initially zero, the first         operand address designates the location of the 256 byte         transaction diagnostic block, called the TBEGIN-specified TDB         (described further below) into which various diagnostic         information may be stored if the transaction is aborted. When         the CPU is in the primary space mode or access register mode,         the first operand address designates a location in the primary         address space. When the CPU is in the secondary space or home         space mode, the first operand address designates a location in         the secondary or home address space, respectively. When DAT is         off, the transaction diagnostic block (TDB) address (TDBA)         designates a location in real storage.     -   Store accessibility to the first operand is determined. If         accessible, the logical address of the operand is placed into         the transaction diagnostic block address (TDBA), and the TDBA is         valid.     -   When the CPU is already in the non-constrained transactional         execution mode, the TDBA is not modified, and it is         unpredictable whether the first operand is tested for         accessibility.

When the B₁ field is zero, no access exceptions are detected for the first operand and, for the outermost TBEGIN instruction, the TDBA is invalid.

The bits of the I₂ field are defined as follows, in one example:

General Register Save Mask (GRSM) 210 (FIG. 2B): Bits 0-7 of the I₂ field contain the general register save mask (GRSM). Each bit of the GRSM represents an even-odd pair of general registers, where bit 0 represents registers 0 and 1, bit 1 represents registers 2 and 3, and so forth. When a bit in the GRSM of the outermost TBEGIN instruction is zero, the corresponding register pair is not saved. When a bit in the GRSM of the outermost TBEGIN instruction is one, the corresponding register pair is saved in a model dependent location that is not directly accessible by the program.

If the transaction aborts, saved register pairs are restored to their contents when the outermost TBEGIN instruction was executed. The contents of all other (unsaved) general registers are not restored when a transaction aborts.

The general register save mask is ignored on all TBEGINs except for the outermost one.

Allow AR Modification (A) 212: The A control, bit 12 of the I₂ field, controls whether the transaction is allowed to modify an access register. The effective allow AR modification control is the logical AND of the A control in the TBEGIN instruction for the current nesting level and for all outer levels.

If the effective A control is zero, the transaction will be aborted with abort code 11 (restricted instruction) if an attempt is made to modify any access register. If the effective A control is one, the transaction will not be aborted if an access register is modified (absent of any other abort condition).

Allow Floating Point Operation (F) 214: The F control, bit 13 of the I₂ field, controls whether the transaction is allowed to execute specified floating point instructions. The effective allow floating point operation control is the logical AND of the F control in the TBEGIN instruction for the current nesting level and for all outer levels.

If the effective F control is zero, then (a) the transaction will be aborted with abort code 11 (restricted instruction) if an attempt is made to execute a floating point instruction, and (b) the data exception code (DXC) in byte 2 of the floating point control register (FPCR) will not be set by any data exception program exception condition. If the effective F control is one, then (a) the transaction will not be aborted if an attempt is made to execute a floating point instruction (absent any other abort condition), and (b) the DXC in the FPCR may be set by a data exception program exception condition.

Program Interruption Filtering Control (PIFC) 216: Bits 14-15 of the I₂ field are the program interruption filtering control (PIFC). The PIFC controls whether certain classes of program exception conditions (e.g., addressing exception, data exception, operation exception, protection exception, etc.) that occur while the CPU is in the transactional execution mode result in an interruption.

The effective PIFC is the highest value of the PIFC in the TBEGIN instruction for the current nesting level and for all outer levels. When the effective PIFC is zero, all program exception conditions result in an interruption. When the effective PIFC is one, program exception conditions having a transactional execution class of 1 and 2 result in an interruption. (Each program exception condition is assigned at least one transactional execution class, depending on the severity of the exception. Severity is based on the likelihood of recovery during a repeated execution of the transactional execution, and whether the operating system needs to see the interruption.) When the effective PIFC is two, program exception conditions having a transactional execution class of 1 result in an interruption. A PIFC of 3 is reserved.

Bits 8-11 of the I₂ field (bits 40-43 of the instruction) are reserved and should contain zeros; otherwise, the program may not operate compatibly in the future.

One embodiment of a format of a Transaction Begin constrained (TBEGINC) instruction is described with reference to FIGS. 3A-3B. In one example, TBEGINC 300 (FIG. 3A) includes an opcode field 302 that includes an opcode specifying a transaction begin constrained operation; a base field (B₁) 304; a displacement field (D₁) 306; and an immediate field (I₂) 308. The contents of the general register specified by B₁ 304 are added to D₁ 306 to obtain the first operand address. However, with the transaction begin constrained instruction, the first operand address is not used to access storage. Instead, the B₁ field of the instruction includes zeros; otherwise, a specification exception is recognized.

In one embodiment, the I₂ field includes various controls, an example of which is depicted in FIG. 3B.

The bits of the I₂ field are defined as follows, in one example:

-   -   General Register Save Mask (GRSM) 310: Bits 0-7 of the I₂ field         contain the general register save mask (GRSM). Each bit of the         GRSM represents an even-odd pair of general registers, where bit         0 represents registers 0 and 1, bit 1 represents registers 2 and         3, and so forth. When a bit in the GRSM is zero, the         corresponding register pair is not saved. When a bit in the GRSM         is one, the corresponding register pair is saved in a         model-dependent location that is not directly accessible by the         program.     -   If the transaction aborts, saved register pairs are restored to         their contents when the outermost TRANSACTION BEGIN instruction         was executed. The contents of all other (unsaved) general         registers are not restored when a constrained transaction         aborts.     -   When TBEGINC is used to continue execution in the         non-constrained transaction execution mode, the general register         save mask is ignored.     -   Allow AR Modification (A) 312: The A control, bit 12 of the I₂         field, controls whether the transaction is allowed to modify an         access register. The effective allow-AR-modification control is         the logical AND of the A control in the TBEGINC instruction for         the current nesting level and for any outer TBEGIN or TBEGINC         instructions.     -   If the effective A control is zero, the transaction will be         aborted with abort code 11 (restricted instruction) if an         attempt is made to modify any access register. If the effective         A control is one, the transaction will not be aborted if an         access register is modified (absent of any other abort         condition).     -   Bits 8-11 and 13-15 of the I₂ field (bits 40-43 and 45-47 of the         instruction) are reserved and should contain zeros.

The end of a Transaction Begin instruction is specified, in one example, by a TRANSACTION END (TEND) instruction, a format of which is depicted in FIG. 4. As one example, a TEND instruction 400 includes an opcode field 402 that includes an opcode specifying a transaction end operation.

A number of terms are used with respect to the transactional execution facility, and therefore, solely for convenience, a list of terms is provided below in alphabetical order. In one embodiment, these terms have the following definition:

Abort: A transaction aborts when it is ended prior to a TRANSACTION END instruction that results in a transaction nesting depth of zero, or when a CONDITIONAL TRANSACTION END instruction sets a condition code of zero. When a transaction aborts, the following occurs, in one embodiment:

-   -   Transactional store accesses made by any and all levels of the         transaction are discarded (that is, not committed).     -   Non-transactional store accesses made by any and all levels of         the transaction are committed.     -   Registers designated by the general register save mask (GRSM) of         the outermost TRANSACTION BEGIN instruction are restored to         their contents prior to the transactional execution (that is, to         their contents at execution of the outermost TRANSACTION BEGIN         instruction). General registers not designated by the general         register save mask of the outermost TRANSACTION BEGIN         instruction are not restored.     -   Access registers, floating-point registers, and the         floating-point control register are not restored. Any changes         made to these registers during transaction execution are         retained when the transaction aborts.

A transaction may be aborted due to a variety of reasons, including attempted execution of a restricted instruction, attempted modification of a restricted resource, transactional conflict, exceeding various CPU resources, any interpretive-execution interception condition, any interruption, a TRANSACTION ABORT instruction, and other reasons. A transaction-abort code provides specific reasons why a transaction may be aborted.

One example of a format of a TRANSACTION ABORT (TABORT) instruction is described with reference to FIG. 5. As one example, a TABORT instruction 500 includes an opcode field 502 that includes an opcode specifying a transaction abort operation; a base field (B₂) 504; and a displacement field (D₂) 506. When the B₂ field is nonzero, the contents of the general register specified by B₂ 504 are added to D₂ 506 to obtain a second operand address; otherwise, the second operand address is formed solely from the D₂ field, and the B₂ field is ignored. The second operand address is not used to address data; instead, the address forms the transaction abort code which is placed in a transaction diagnostic block during abort processing. Address computation for the second operand address follows the rules of address arithmetic: in the 24-bit addressing mode, bits 0-29 are set to zeros; in the 31-bit addressing mode, bits 0-32 are set to zeros.

Commit: At the completion of an outermost TRANSACTION END instruction, or at the completion of a CONDITIONAL TRANSACTION END instruction that sets condition code 0, the CPU commits the store accesses made by the transaction (i.e., the outermost transaction and any nested levels) such that they are visible to other CPUs and the I/O subsystem. As observed by other CPUs and by the I/O subsystem, all fetch and store accesses made by all nested levels of the transaction appear to occur as a single concurrent operation when the commit occurs.

The contents of the general registers, access registers, floating-point registers, and the floating-point control register are not modified by the commit process. Any changes made to these registers during transactional execution are retained when the transaction's stores are committed.

Conflict: A transactional access made by one CPU conflicts with either (a) a transactional access or non-transactional access made by another CPU, or (b) the non-transactional access made by the I/O subsystem, if both accesses are to any location within the same cache line, and one or more of the accesses is a store.

A conflict may be detected by a CPU's speculative execution of instructions, even though the conflict may not be detected in the conceptual sequence.

Constrained Transaction: A constrained transaction is a transaction that executes in the constrained transactional execution mode and is subject to the following limitations:

-   -   A subset of the general instructions is available.     -   A limited number of instructions may be executed.     -   A limited number of storage-operand locations may be accessed.     -   The transaction is limited to a single nesting level.

In the absence of repeated interruptions or conflicts with other CPUs or the I/O subsystem, a constrained transaction eventually completes, thus an abort-handler routine is not required.

When a TRANSACTION BEGIN constrained (TBEGINC) instruction is executed while the CPU is already in the non-constrained transaction execution mode, execution continues as a nested non-constrained transaction.

Constrained Transactional Execution Mode: When the transaction nesting depth is zero, and a transaction is initiated by a TBEGINC instruction, the CPU enters the constrained transactional execution mode. While the CPU is in the constrained transactional execution mode, the transaction nesting depth is one.

Nested Transaction: When the TRANSACTION BEGIN instruction is issued while the CPU is in the non-constrained transactional execution mode, the transaction is nested.

The transactional execution facility uses a model called flattened nesting. In the flattened nesting mode, stores made by an inner transaction are not observable by other CPUs and by the I/O subsystem until the outermost transaction commits its stores. Similarly, if a transaction aborts, all nested transactions abort, and all transactional stores of all nested transactions are discarded.

One example of nested transactions is depicted in FIG. 6. As shown, a first TBEGIN 600 starts an outermost transaction 601, TBEGIN 602 starts a first nested transaction, and TBEGIN 604 starts a second nested transaction. In this example, TBEGIN 604 and TEND 606 define an innermost transaction 608. When TEND 610 executes, transactional stores are committed 612 for the outermost transaction and all inner transactions.

Non-Constrained Transaction: A non-constrained transaction is a transaction that executes in the non-constrained transactional execution mode. Although a non-constrained transaction is not limited in the manner as a constrained transaction, it may still be aborted due to a variety of causes.

Non-Constrained Transactional Execution Mode: When a transaction is initiated by the TBEGIN instruction, the CPU enters the non-constrained transactional execution mode. While the CPU is in the non-constrained transactional execution mode, the transaction nesting depth may vary from one to the maximum transaction nesting depth.

Non-Transactional Access: Non-transactional accesses are storage operand accesses made by the CPU when it is not in the transactional execution mode (that is, classic storage accesses outside of a transaction). Further, accesses made by the I/O subsystem are non-transactional accesses. Additionally, the NONTRANSACTIONAL STORE instruction may be used to cause a non-transactional store access while the CPU is in the non-constrained transactional execution mode.

Outer/Outermost Transaction: A transaction with a lower-numbered transaction nesting depth is an outer transaction. A transaction with a transaction nesting depth value of one is the outermost transaction.

An outermost TRANSACTION BEGIN instruction is one that is executed when the transaction nesting depth is initially zero. An outermost TRANSACTION END instruction is one that causes the transaction nesting depth to transition from one to zero. Further, a CONDITIONAL TRANSACTION END instruction that sets the condition code to zero may also be considered to be the outermost form of the instruction. A constrained transaction is the outermost transaction, in this embodiment.

Program Interruption Filtering: When a transaction is aborted due to certain program exception conditions, the program can optionally prevent the interruption from occurring. This technique is called program-interruption filtering. Program interruption filtering is subject to the transactional class of the interruption, the effective program interruption filtering control from the TRANSACTION BEGIN instruction, and the transactional execution program interruption filtering override in control register 0.

Transaction: A transaction includes the storage-operand accesses made, and selected general registers altered, while the CPU is in the transaction execution mode. For a non-constrained transaction, storage-operand accesses may include both transactional accesses and non-transactional accesses. For a constrained transaction, storage-operand accesses are limited to transactional accesses. As observed by other CPUs and by the I/O subsystem, all storage-operand accesses made by the CPU while in the transaction execution mode appear to occur as a single concurrent operation. If a transaction is aborted, transactional store accesses are discarded, and any registers designated by the general register save mask of the outermost TRANSACTION BEGIN instruction are restored to their contents prior to transactional execution.

Transactional Accesses: Transactional accesses are storage operand accesses made while the CPU is in the transactional execution mode, with the exception of accesses made by the NONTRANSACTIONAL STORE instruction.

Transactional Execution Mode: The term transactional execution mode (a.k.a., transaction execution mode) describes the common operation of both the non-constrained and the constrained transactional execution modes. Thus, when the operation is described, the terms non-constrained and constrained are used to qualify the transactional execution mode.

When the transaction nesting depth is zero, the CPU is not in the transactional execution mode (also called the non-transactional execution mode).

As observed by the CPU, fetches and stores made in the transactional execution mode are no different than those made while not in the transactional execution mode.

In one embodiment of the z/Architecture, the transactional execution facility is under the control of bits 8-9 of control register 0, bits 61-63 of control register 2, the transaction nesting depth, the transaction diagnostic block address, and the transaction abort program status word (PSW).

Following an initial CPU reset, the contents of bit positions 8-9 of control register 0, bit positions 62-63 of control register 2, and the transaction nesting depth are set to zero. When the transactional execution control, bit 8 of control register 0, is zero, the CPU cannot be placed into the transactional execution mode.

Further details regarding the various controls are described below.

As indicated, the transactional execution facility is controlled by two bits in control register zero and three bits in control register two. For instance:

-   -   Control Register 0 Bits: The bit assignments are as follows, in         one embodiment:         -   Transactional Execution Control (TXC): Bit 8 of control             register zero is the transactional execution control. This             bit provides a mechanism whereby the control program (e.g.,             operating system) can indicate whether or not the             transactional execution facility is usable by the program.             Bit 8 is to be one to successfully enter the transactional             execution mode.         -   When bit 8 of control register 0 is zero, attempted             execution of the CONDITIONAL TRANSACTION END, EXTRACT             TRANSACTION NESTING DEPTH, TRANSACTION BEGIN and TRANSACTION             END instructions results in a special operation execution.         -   Transaction Execution Program Interruption Filtering             Override (PIFO): Bit 9 of control register zero is the             transactional execution program interruption filtering             override. This bit provides a mechanism by which the control             program can ensure that any program exception condition that             occurs while the CPU is in the transactional execution mode             results in an interruption, regardless of the effective             program interruption filtering control specified or implied             by the TRANSACTION BEGIN instruction(s).

Control Register 2 Bits: The assignments are as follows, in one embodiment:

-   -   Transaction Diagnostic Scope (TDS): Bit 61 of control register 2         controls the applicability of the transaction diagnosis control         (TDC) in bits 62-63 of the register, as follows:

TDS

Value Meaning 0 The TDC applies regardless of whether the CPU is in the problem or supervisor state. 1 The TDC applies only when the CPU is in the problem state. When the CPU is in the supervisor state, processing is as if the TDC contained zero.

-   -   Transaction Diagnostic Control (TDC): Bits 62-63 of control         register 2 are a 2-bit unsigned integer that may be used to         cause transactions to be randomly aborted for diagnostic         purposes. The encoding of the TDC is as follows, in one example:

TDC

Value Meaning 0 Normal operation; transactions are not aborted as a result of the TDC. 1 Abort every transaction at a random instruction, but before execution of the outermost TRANSACTION END instruction or CONDITIONAL TRANSACTION END instruction that sets condition code zero. 2 Abort random transactions at a random instruction. 3 Reserved

When a transaction is aborted due to a nonzero TDC, then either of the following may occur:

-   -   The abort code is set to any of the codes 7-11, 13-18, or 255,         with the value of the code randomly chosen by the CPU; the         condition code is set corresponding to the abort code.     -   For a non-constrained transaction, the condition code is set to         one. In this case, the abort code is not applicable.

It is model dependent whether TDC value 1 is implemented. If not implemented, a value of 1 acts as if 2 was specified.

For a constrained transaction, a TDC value of 1 is treated as if a TDC value of 2 was specified.

If a TDC value of 3 is specified, the results are unpredictable.

Transaction Diagnostic Block Address (TDBA)

A valid transaction diagnostic block address (TDBA) is set from the first operand address of the outermost TRANSACTION BEGIN (TBEGIN) instruction when the B₁ field of the instruction is nonzero. When the CPU is in the primary space or access register mode, the TDBA designates a location in the primary address space. When the CPU is in the secondary space, or home space mode, the TDBA designates a location in the secondary or home address space, respectively. When DAT (Dynamic Address Translation) is off, the TDBA designates a location in real storage.

The TDBA is used by the CPU to locate the transaction diagnostic block—called the TBEGIN-specified TDB—if the transaction is subsequently aborted. The rightmost three bits of the TDBA are zero, meaning that the TBEGIN-specified TDB is on a doubleword boundary.

When the B₁ field of an outermost TRANSACTION BEGIN (TBEGIN) instruction is zero, the transactional diagnostic block address is invalid, and no TBEGIN-specified TDB is stored if the transaction is subsequently aborted.

Transaction Abort PSW (TAPSW)

During execution of the TRANSACTION BEGIN (TBEGIN) instruction when the nesting depth is initially zero, the transaction abort PSW is set to the contents of the current PSW; and the instruction address of the transaction abort PSW designates the next sequential instruction (that is, the instruction following the outermost TBEGIN). During execution of the TRANSACTION BEGIN constrained (TBEGINC) instruction when the nesting depth is initially zero, the transaction abort PSW is set to the contents of the current PSW, except that the instruction address of the transaction abort PSW designates the TBEGINC instruction (rather than the next sequential instruction following the TBEGINC).

When a transaction is aborted, the condition code in the transaction abort PSW is replaced with a code indicating the severity of the abort condition. Subsequently, if the transaction was aborted due to causes that do not result in an interruption, the PSW is loaded from the transaction abort PSW; if the transaction was aborted due to causes that result in an interruption, the transaction abort PSW is stored as the interruption old PSW.

The transaction abort PSW is not altered during the execution of any inner TRANSACTION BEGIN instruction.

Transaction Nesting Depth (TND)

The transaction nesting depth is, for instance, a 16-bit unsigned value that is incremented each time a TRANSACTION BEGIN instruction is completed with condition code 0 and decremented each time a TRANSACTION END or CONDITIONAL TRANSACTION END instruction is completed with condition code zero. The transaction nesting depth is reset to zero when a transaction is aborted or by CPU reset.

In one embodiment, a maximum TND of 15 is implemented.

In one implementation, when the CPU is in the constrained transactional execution mode, the transaction nesting depth is one. Additionally, although the maximum TND can be represented as a 4-bit value, the TND is defined to be a 16-bit value to facilitate its inspection in the transaction diagnostic block.

Transaction Diagnostic Block (TDB)

When a transaction is aborted, various status information may be saved in a transaction diagnostic block (TDB), as follows:

-   -   1. TBEGIN-specified TDB: For a non-constrained transaction, when         the B₁ field of the outermost TBEGIN instruction is nonzero, the         first operand address of the instruction designates the         TBEGIN-specified TDB. This is an application program specified         location that may be examined by the application's abort         handler.     -   2. Program-Interruption (PI) TDB: If a non-constrained         transaction is aborted due to a non-filtered program exception         condition, or if a constrained transaction is aborted due to any         program exception condition (that is, any condition that results         in a program interruption being recognized), the PI-TDB is         stored into locations in the prefix area. This is available for         the operating system to inspect and log out in any diagnostic         reporting that it may provide.     -   3. Interception TDB: If the transaction is aborted due to any         program exception condition that results in interception (that         is, the condition causes interpretive execution to end and         control to return to the host program), a TDB is stored into a         location specified in the state description block for the guest         operating system.

The TBEGIN-specified TDB is only stored, in one embodiment, when the TDB address is valid (that is, when the outermost TBEGIN instruction's B₁ field is nonzero).

For aborts due to unfiltered program exception conditions, only one of either the PI-TDB or Interception TDB will be stored. Thus, there may be zero, one, or two TDBs stored for an abort.

Further details regarding one example of each of the TDBs are described below:

TBEGIN-Specified TDB: The 256-byte location specified by a valid transaction diagnostic block address. When the transaction diagnostic block address is valid, the TBEGIN-specified TDB is stored on a transaction abort. The TBEGIN-specified TDB is subject to all storage protection mechanisms that are in effect at the execution of the outermost TRANSACTION BEGIN instruction. A PER (Program Event Recording) storage alteration event for any portion of the TBEGIN-specified TDB is detected during the execution of the outermost TBEGIN, not during the transaction abort processing.

One purpose of PER is to assist in debugging programs. It permits the program to be alerted to the following types of events, as examples:

-   -   Execution of a successful branch instruction. The option is         provided of having an event occur only when the branch target         location is within the designated storage area.     -   Fetching of an instruction from the designated storage area.     -   Alteration of the contents of the designated storage area. The         option is provided of having an event occur only when the         storage area is within designated address spaces.     -   Execution of a STORE USING REAL ADDRESS instruction.     -   Execution of the CONDITIONAL TRANSACTION END or TRANSACTION END         instruction.

The program can selectively specify that one or more of the above types of events be recognized, except that the event for STORE USING REAL ADDRESS can be specified only along with the storage alteration event. The information concerning a PER event is provided to the program by means of a program interruption, with the cause of the interruption being identified in the interruption code.

When the transaction diagnostic block address is not valid, a TBEGIN-specified TDB is not stored.

Program-Interruption TDB: Real locations 6,144-6,399 (1800-18FF hex). The program interruption TDB is stored when a transaction is aborted due to program interruption. When a transaction is aborted due to other causes, the contents of the program interruption TDB are unpredictable.

The program interruption TDB is not subject to any protection mechanism. PER storage alteration events are not detected for the program interruption TDB when it is stored during a program interruption.

Interception TDB: The 256-byte host real location specified by locations 488-495 of the state description. The interception TDB is stored when an aborted transaction results in a guest program interruption interception (that is, interception code 8). When a transaction is aborted due to other causes, the contents of the interception TDB are unpredictable. The interception TDB is not subject to any protection mechanism.

As depicted in FIG. 7, the fields of a transaction diagnostic block 700 are as follows, in one embodiment:

Format 702: Byte 0 contains a validity and format indication, as follows:

Value Meaning 0 The remaining fields of the TDB are unpredictable. 1 A format-1 TDB, the remaining fields of which are described below. 2-255 Reserved

A TDB in which the format field is zero is referred to as a null TDB.

Flags 704: Byte 1 contains various indications, as follows:

-   -   Conflict Token Validity (CTV): When a transaction is aborted due         to a fetch or store conflict (that is, abort codes 9 or 10,         respectively), bit 0 of byte 1 is the conflict token validity         indication. When the CTV indication is one, the conflict token         710 in bytes 16-23 of the TDB contain the logical address at         which the conflict was detected. When the CTV indication is         zero, bytes 16-23 of the TDB are unpredictable.     -   When a transaction is aborted due to any reason other than a         fetch or store conflict, bit 0 of byte 1 is stored as zero.     -   Constrained-Transaction Indication (CTI): When the CPU is in the         constrained transactional execution mode, bit 1 of byte 1 is set         to one. When the CPU is in the non-constrained transactional         execution mode, bit 1 of byte 1 is set to zero.     -   Reserved: Bits 2-7 of byte 1 are reserved, and stored as zeros.     -   Transaction Nesting Depth (TND) 706: Bytes 6-7 contain the         transaction nesting depth when the transaction was aborted.     -   Transaction Abort Code (TAC) 708: Bytes 8-15 contain a 64-bit         unsigned transaction abort code. Each code indicates a reason         for a transaction being aborted.     -   It is model dependent whether the transaction abort code is         stored in the program interruption TDB when a transaction is         aborted due to conditions other than a program interruption.     -   Conflict Token 710: For transactions that are aborted due to         fetch or store conflict (that is, abort codes 9 and 10,         respectively), bytes 16-23 contain the logical address of the         storage location at which the conflict was detected. The         conflict token is meaningful when the CTV bit, bit 0 of byte 1,         is one.     -   When the CTV bit is zero, bytes 16-23 are unpredictable.     -   Because of speculative execution by the CPU, the conflict token         may designate a storage location that would not necessarily be         accessed by the transaction's conceptual execution sequence.     -   Aborted Transaction Instruction Address (ATIA) 712: Bytes 24-31         contain an instruction address that identifies the instruction         that was executing when an abort was detected. When a         transaction is aborted due to abort codes 2, 5, 6, 11, 13, 17,         18, or 256 or higher, or when a transaction is aborted due to         abort codes 4 or 12 and the program exception condition is         nullifying, the ATIA points directly to the instruction that was         being executed. When a transaction is aborted due to abort codes         4 or 12, and the program exception condition is not nullifying,         the ATIA points past the instruction that was being executed.     -   When a transaction is aborted due to abort codes 7-10, 14-16, or         255, the ATIA does not necessarily indicate the exact         instruction causing the abort, but may point to an earlier or         later instruction within the transaction.     -   If a transaction is aborted due to an instruction that is the         target of an execute-type instruction, the ATIA identifies the         execute-type instruction, either pointing to the instruction or         past it, depending on the abort code. The ATIA does not indicate         the target of the execute-type instruction.     -   The ATIA is subject to the addressing mode when the transaction         is aborted. In the 24-bit addressing mode, bits 0-40 of the         field contain zeros. In the 31-bit addressing mode, bits 0-32 of         the field contain zeros.     -   It is model dependent whether the aborted transaction         instruction address is stored in the program interruption TDB         when a transaction is aborted due to conditions other than a         program interruption.     -   When a transaction is aborted due to abort code 4 or 12, and the         program exception condition is not nullifying, the ATIA does not         point to the instruction causing the abort. By subtracting the         number of halfwords indicated by the interruption length code         (ILC) from the ATIA, the instruction causing the abort can be         identified in conditions that are suppressing or terminating, or         for non-PER events that are completing. When a transaction is         aborted due to a PER event, and no other program exception         condition is present, the ATIA is unpredictable.     -   When the transaction diagnostic block address is valid, the ILC         may be examined in program interruption identification (PIID) in         bytes 36-39 of the TBEGIN-specified TDB. When filtering does not         apply, the ILC may be examined in the PIID at location 140-143         in real storage.     -   Exception Access Identification (EAID) 714: For transactions         that are aborted due to certain filtered program exception         conditions, byte 32 of the TBEGIN-specified TDB contains the         exception access identification. In one example of the         z/Architecture, the format of the EAID, and the cases for which         it is stored, are the same as those described in real location         160 when the exception condition results in an interruption, as         described in the above-incorporated by reference Principles of         Operation.     -   For transactions that are aborted for other reasons, including         any exception conditions that result in a program interruption,         byte 32 is unpredictable. Byte 32 is unpredictable in the         program interruption TDB.     -   This field is stored only in the TDB designated by the         transaction diagnostic block address; otherwise, the field is         reserved. The EAID is stored only for access list controlled or         DAT protection, ASCE-type, page translation, region first         translation, region second translation, region third         translation, and segment translation program exception         conditions.     -   Data Exception Code (DXC) 716: For transactions that are aborted         due to filtered data exception program exception conditions,         byte 33 of the TBEGIN specified TDB contains the data exception         code. In one example of the z/Architecture, the format of the         DXC, and the cases for which it is stored, are the same as those         described in real location 147 when the exception condition         results in an interruption, as described in the         above-incorporated by reference Principles of Operation. In one         example, location 147 includes the DXC.     -   For transactions that are aborted for other reasons, including         any exception conditions that result in a program interruption,         byte 33 is unpredictable. Byte 33 is unpredictable in the         program interruption TDB.     -   This field is stored only in the TDB designated by the         transaction diagnostic block address; otherwise, the field is         reserved. The DXC is stored only for data program exception         conditions.     -   Program Interruption Identification (PIID) 718: For transactions         that are aborted due to filtered program exception conditions,         bytes 36-39 of the TBEGIN-specified TDB contain the program         interruption identification. In one example of the         z/Architecture, the format of the PIID is the same as that         described in real locations 140-143 when the condition results         in an interruption (as described in the above-incorporated by         reference Principles of Operation), except that the instruction         length code in bits 13-14 of the PIID is respective to the         instruction at which the exception condition was detected.     -   For transactions that are aborted for other reasons, including         exception conditions that result in a program interruption,         bytes 36-39 are unpredictable. Bytes 36-39 are unpredictable in         the program interruption TDB.     -   This field is stored only in the TDB designated by the         transaction diagnostic block address; otherwise, the field is         reserved. The program interruption identification is only stored         for program exception conditions.     -   Translation Exception Identification (TEID) 720: For         transactions that are aborted due to any of the following         filtered program exception conditions, bytes 40-47 of the         TBEGIN-specified TDB contain the translation exception         identification.         -   Access list controlled or DAT protection         -   ASCE-type         -   Page translation         -   Region-first translation         -   Region-second translation         -   Region-third translation         -   Segment translation exception     -   In one example of the z/Architecture, the format of the TED is         the same as that described in real locations 168-175 when the         condition results in an interruption, as described in the         above-incorporated by reference Principles of Operation.     -   For transactions that are aborted for other reasons, including         exception conditions that result in a program interruption,         bytes 40-47 are unpredictable. Bytes 40-47 are unpredictable in         the program interruption TDB.     -   This field is stored only in the TDB designated by the         transaction diagnostic block address; otherwise, the field is         reserved.     -   Breaking Event Address 722: For transactions that are aborted         due to filtered program exception conditions, bytes 48-55 of the         TBEGIN-specified TDB contain the breaking event address. In one         example of the z/Architecture, the format of the breaking event         address is the same as that described in real locations 272-279         when the condition results in an interruption, as described in         the above-incorporated by reference Principles of Operation.     -   For transactions that are aborted for other reasons, including         exception conditions that result in a program interruption,         bytes 48-55 are unpredictable. Bytes 48-55 are unpredictable in         the program interruption TDB.     -   This field is stored only in the TDB designated by the         transaction diagnostic block address; otherwise, the field is         reserved.

Further details relating to breaking events are described below.

-   -   In one embodiment of the z/Architecture, when the PER-3 facility         is installed, it provides the program with the address of the         last instruction to cause a break in the sequential execution of         the CPU. Breaking event address recording can be used as a         debugging assist for wild branch detection. This facility         provides, for instance, a 64-bit register in the CPU, called the         breaking event address register. Each time an instruction other         than TRANSACTION ABORT causes a break in the sequential         instruction execution (that is, the instruction address in the         PSW is replaced, rather than incremented by the length of the         instruction), the address of that instruction is placed in the         breaking event address register. Whenever a program interruption         occurs, whether or not PER is indicated, the current contents of         the breaking event address register are placed in real storage         locations 272-279.     -   If the instruction causing the breaking event is the target of         an execute-type instruction (EXECUTE or EXECUTE RELATIVE LONG),         then the instruction address used to fetch the execute-type         instruction is placed in the breaking event address register.     -   In one embodiment of the z/Architecture, a breaking event is         considered to occur whenever one of the following instructions         causes branching: BRANCH AND LINK (BAL, BALR); BRANCH AND SAVE         (BAS, BASR); BRANCH AND SAVE AND SET MODE (BASSM); BRANCH AND         SET MODE (BSM); BRANCH AND STACK (BAKR); BRANCH ON CONDITION         (BC, BCR); BRANCH ON COUNT (BCT, BCTR, BCTG, BCTGR); BRANCH ON         INDEX HIGH (BXH, BXHG); BRANCH ON INDEX LOW OR EQUAL (BXLE,         BXLEG); BRANCH RELATIVE ON CONDITION (BRC); BRANCH RELATIVE ON         CONDITION LONG (BRCL); BRANCH RELATIVE ON COUNT (BRCT, BRCTG);         BRANCH RELATIVE ON INDEX HIGH (BRXH, BRXHG); BRANCH RELATIVE ON         INDEX LOW OR EQUAL (BRXLE, BRXLG); COMPARE AND BRANCH (CRB,         CGRB); COMPARE AND BRANCH RELATIVE (CRJ, CGRJ); COMPARE         IMMEDIATE AND BRANCH (CIB, CGIB); COMPARE IMMEDIATE AND BRANCH         RELATIVE (CIJ, CGIJ); COMPARE LOGICAL AND BRANCH (CLRB, CLGRB);         COMPARE LOGICAL AND BRANCH RELATIVE (CLRJ, CLGRJ); COMPARE         LOGICAL IMMEDIATE AND BRANCH (CLIB, CLGIB); and COMPARE LOGICAL         IMMEDIATE AND BRANCH RELATIVE (CLU, CLGIJ).     -   A breaking event is also considered to occur whenever one of the         following instructions completes: BRANCH AND SET AUTHORITY         (BSA); BRANCH IN SUBSPACE GROUP (BSG); BRANCH RELATIVE AND SAVE;         BRANCH RELATIVE AND SAVE LONG (BRASL); LOAD PSW (LPSW); LOAD PSW         EXTENDED (LPSWE); PROGRAM CALL (PC); PROGRAM RETURN (PR);         PROGRAM TRANSFER (PT); PROGRAM TRANSFER WITH INSTANCE (PTI);         RESUME PROGRAM (RP); and TRAP (TRAP2, TRAP4).     -   A breaking event is not considered to occur as a result of a         transaction being aborted (either implicitly or as a result of         the TRANSACTION ABORT instruction).     -   Model Dependent Diagnostic Information 724: Bytes 112-127         contain model dependent diagnostic information.     -   For all abort codes except 12 (filtered program interruption),         the model dependent diagnostic information is saved in each TDB         that is stored.     -   In one embodiment, the model dependent diagnostic information         includes the following:         -   Bytes 112-119 contain a vector of 64 bits called the             transactional execution branch indications (TXBI). Each of             the first 63 bits of the vector indicates the results of             executing a branching instruction while the CPU was in the             transactional execution mode, as follows:

Value Meaning 0 The instruction completed without branching. 1 The instruction completed with branching.

-   -   -   Bit 0 represents the result of the first such branching             instruction; bit 1 represents the result of the second such             instruction, and so forth.         -   If fewer than 63 branching instructions were executed while             the CPU was in the transactional execution mode, the             rightmost bits that do not correspond to branching             instructions are set to zeros (including bit 63). When more             than 63 branching instructions were executed, bit 63 of the             TXBI is set to one.         -   Bits in the TXBI are set by instructions which are capable             of causing a breaking event, as listed above, except for the             following:             -   Any restricted instruction does not cause a bit to be                 set in the TXBI.             -   For instructions of, for instance, the z/Architecture,                 when the M₁ field of the BRANCH ON CONDITION, BRANCH                 RELATIVE ON CONDITION, or BRANCH RELATIVE ON CONDITION                 LONG instruction is zero, or when the R₂ field of the                 following instructions is zero, it is model dependent                 whether the execution of the instruction causes a bit to                 be set in the TXBI.                 -   BRANCH AND LINK (BALR); BRANCH AND SAVE (BASR);                     BRANCH AND SAVE AND SET MODE (BASSM); BRANCH AND SET                     MODE (BSM); BRANCH ON CONDITION (BCR); and BRANCH ON                     COUNT (BCTR, BCTGR)

    -   For abort conditions that were caused by a host access         exception, bit position 0 of byte 127 is set to one. For all         other abort conditions, bit position 0 of byte 127 is set to         zero.

    -   For abort conditions that were detected by the load/store unit         (LSU), the rightmost five bits of byte 127 contain an indication         of the cause. For abort conditions that were not detected by the         LSU, byte 127 is reserved.

    -   General Registers 730: Bytes 128-255 contain the contents of         general registers 0-15 at the time the transaction was aborted.         The registers are stored in ascending order, beginning with         general register 0 in bytes 128-135, general register 1 in bytes         136-143, and so forth.

    -   Reserved: All other fields are reserved. Unless indicated         otherwise, the contents of reserved fields are unpredictable.

As observed by other CPUs and the I/O subsystem, storing of the TDB(s) during a transaction abort is a multiple access reference occurring after any non-transactional stores.

A transaction may be aborted due to causes that are outside the scope of the immediate configuration in which it executes. For example, transient events recognized by a hypervisor (such as LPAR or z/VM) may cause a transaction to be aborted.

The information provided in the transaction diagnostic block is intended for diagnostic purposes and is substantially correct. However, because an abort may have been caused by an event outside the scope of the immediate configuration, information such as the abort code or program interruption identification may not accurately reflect conditions within the configuration, and thus, should not be used in determining program action.

In addition to the diagnostic information saved in the TDB, when a transaction is aborted due to any data exception program exception condition and both the AFP register control, bit 45 of control register 0, and the effective allow floating point operation control (F) are one, the data exception code (DXC) is placed into byte 2 of the floating point control register (FPCR), regardless of whether filtering applies to the program exception condition. When a transaction is aborted, and either or both the AFP register control or effective allow floating point operation control are zero, the DXC is not placed into the FPCR.

In one embodiment, as indicated herein, when the transaction execution facility is installed, a number of general instructions are provided, including, for instance, EXTRACT TRANSACTION NESTING DEPTH, NONTRANSACTIONAL STORE, TRANSACTION ABORT, TRANSACTION BEGIN and TRANSACTION END. Further, when the conditional transaction end facility is installed, the CONDITIONAL TRANSACTION END instruction is provided.

When the CPU is in the transaction execution mode, attempted execution of certain instructions is restricted and causes the transaction to be aborted.

When issued in the constrained transactional execution mode, attempted execution of restricted instructions may also result in a transaction constraint program interruption, or may result in execution proceeding as if the transaction was not constrained.

In one example of the z/Architecture, restricted instructions include, as examples, the following non-privileged instructions: COMPARE AND SWAP AND STORE; MODIFY RUNTIME INSTRUMENTATION CONTROLS; PERFORM LOCKED OPERATION; PREFETCH DATA (RELATIVE LONG), when the code in the M₁ field is 6 or 7; STORE CHARACTERS UNDER MASK HIGH, when the M₃ field is zero and the code in the R₁ field is 6 or 7; STORE FACILITY LIST EXTENDED; STORE RUNTIME INSTRUMENTATION CONTROLS; SUPERVISOR CALL; and TEST RUNTIME INSTRUMENTATION CONTROLS.

In the above list, COMPARE AND SWAP AND STORE and PERFORM LOCKED OPERATION are complex instructions which can be more efficiently implemented by the use of basic instructions in the TX mode. The cases for PREFETCH DATA and PREFETCH DATA RELATIVE LONG are restricted as the codes of 6 and 7 release a cache line, necessitating the commitment of the data potentially prior to the completion of a transaction. SUPERVISOR CALL is restricted as it causes an interruption (which causes a transaction to be aborted).

Under the conditions listed below, the following instructions are restricted:

-   -   BRANCH AND LINK (BALR), BRANCH AND SAVE (BASR), and BRANCH AND         SAVE AND SET MODE, when the R₂ field of the instruction is         nonzero and branch tracing is enabled.     -   BRANCH AND SAVE AND SET MODE and BRANCH AND SET MODE, when the         R₂ field is nonzero and mode tracing is enabled; SET ADDRESSING         MODE, when mode tracing is enabled.     -   CONDITIONAL TRANSACTION END when the transaction nesting depth         is greater than one.     -   MONITOR CALL, when a monitor event condition is recognized.

The above list includes instructions that may form trace entries. If these instructions were allowed to execute transactionally and formed trace entries, and the transaction subsequently aborted, the trace table pointer in control register 12 would be advanced, but the stores to the trace table would be discarded. This would leave an inconsistent gap in the trace table; thus, the instructions are restricted in the cases where they would form trace entries.

When the CPU is in the transactional execution mode, it is model dependent whether the following instructions are restricted: CIPHER MESSAGE; CIPHER MESSAGE WITH CFB; CIPHER MESSAGE WITH CHAINING; CIPHER MESSAGE WITH COUNTER; CIPHER MESSAGE WITH OFB; COMPRESSION CALL; COMPUTE INTERMEDIATE MESSAGE DIGEST; COMPUTE LAST MESSAGE DIGEST; COMPUTE MESSAGE AUTHENTICATION CODE; CONVERT UNICODE-16 TO UNICODE-32; CONVERT UNICODE-16 TO UNICODE-8; CONVERT UNICODE-32 TO UNICODE-16; CONVERT UNICODE-32 TO UNICODE-8; CONVERT UNICODE-8 TO UNICODE-16; CONVERT UNICODE-8 TO UNICODE-32; PERFORM CRYPTOGRAPHIC COMPUTATION; RUNTIME INSTRUMENTATION OFF; and RUNTIME INSTRUMENTATION ON.

Each of the above instructions is either currently implemented by the hardware co-processor, or has been in past machines, and thus, is considered restricted.

When the effective allow AR modification (A) control is zero, the following instructions are restricted: COPY ACCESS; LOAD ACCESS MULTIPLE; LOAD ADDRESS EXTENDED; and SET ACCESS.

Each of the above instructions causes the contents of an access register to be modified. If the A control in the TRANSACTION BEGIN instruction is zero, then the program has explicitly indicated that access register modification is not to be allowed.

When the effective allow floating point operation (F) control is zero, floating point instructions are restricted.

Under certain circumstances, the following instructions may be restricted: EXTRACT CPU TIME; EXTRACT PSW; STORE CLOCK; STORE CLOCK EXTENDED; and STORE CLOCK FAST.

Each of the above instructions is subject to an interception control in the interpretative execution state description. If the hypervisor has set the interception control for these instructions, then their execution may be prolonged due to hypervisor implementation; thus, they are considered restricted if an interception occurs.

When a non-constrained transaction is aborted because of the attempted execution of a restricted instruction, the transaction abort code in the transaction diagnostic block is set to 11 (restricted instruction), and the condition code is set to 3, except as follows: when a non-constrained transaction is aborted due to the attempted execution of an instruction that would otherwise result in a privileged operation exception, it is unpredictable whether the abort code is set to 11 (restricted instruction) or 4 (unfiltered program interruption resulting from the recognition of the privileged operation program interruption). When a non-constrained transaction is aborted due to the attempted execution of PREFETCH DATA (RELATIVE LONG) when the code in the M₁ field is 6 or 7 or STORE CHARACTERS UNDER MASK HIGH when the M₃ field is zero and the code in the R₁ field is 6 or 7, it is unpredictable whether the abort code is set to 11 (restricted instruction) or 16 (cache other). When a non-constrained transaction is aborted due to the attempted execution of MONITOR CALL, and both a monitor event condition and a specification exception condition are present it is unpredictable whether the abort code is set to 11 or 4, or, if the program interruption is filtered, 12.

Additional instructions may be restricted in a constrained transaction. Although these instructions are not currently defined to be restricted in a non-constrained transaction, they may be restricted under certain circumstances in a non-constrained transaction on future processors.

Certain restricted instructions may be allowed in the transactional execution mode on future processors. Therefore, the program should not rely on the transaction being aborted due to the attempted execution of a restricted instruction. The TRANSACTION ABORT instruction should be used to reliably cause a transaction to be aborted.

In a non-constrained transaction, the program should provide an alternative non-transactional code path to accommodate a transaction that aborts due to a restricted instruction.

In operation, when the transaction nesting depth is zero, execution of the TRANSACTION BEGIN (TBEGIN) instruction resulting in condition code zero causes the CPU to enter the non-constrained transactional execution mode. When the transaction nesting depth is zero, execution of the TRANSACTION BEGIN constrained (TBEGINC) instruction resulting in condition code zero causes the CPU to enter the constrained transactional execution mode.

Except where explicitly noted otherwise, all rules that apply for non-transactional execution also apply to transactional execution. Below are additional characteristics of processing while the CPU is in the transactional execution mode.

When the CPU is in the non-constrained transactional execution mode, execution of the TRANSACTION BEGIN instruction resulting in condition code zero causes the CPU to remain in the non-constrained transactional execution mode.

As observed by the CPU, fetches and stores made in the transaction execution mode are no different than those made while not in the transactional execution mode. As observed by other CPUs and by the I/O subsystem, all storage operand accesses made while a CPU is in the transactional execution mode appear to be a single block concurrent access. That is, the accesses to all bytes within a halfword, word, doubleword, or quadword are specified to appear to be block concurrent as observed by other CPUs and I/O (e.g., channel) programs. The halfword, word, doubleword, or quadword is referred to in this section as a block. When a fetch-type reference is specified to appear to be concurrent within a block, no store access to the block by another CPU or I/O program is permitted during the time that bytes contained in the block are being fetched. When a store-type reference is specified to appear to be concurrent within a block, no access to the block, either fetch or store, is permitted by another CPU or I/O program during the time that the bytes within the block are being stored.

Storage accesses for instruction and DAT and ART (Access Register Table) table fetches follow the non-transactional rules.

The CPU leaves the transactional execution mode normally by means of a TRANSACTION END instruction that causes the transaction nesting depth to transition to zero or a CONDITIONAL TRANSACTION END instruction that sets the condition code to zero; in either of these cases, the transaction completes.

When the CPU leaves the transactional execution mode by means of the completion of a TRANSACTION END instruction or a CONDITIONAL TRANSACTION END instruction that sets the condition code to zero, all stores made while in the transactional execution mode are committed; that is, the stores appear to occur as a single block-concurrent operation as observed by other CPUs and by the I/O subsystem.

A transaction may be implicitly aborted for a variety of causes, or it may be explicitly aborted by the TRANSACTION ABORT instruction. Example possible causes of a transaction abort, the corresponding abort code, and the condition code that is placed into the transaction abort PSW are described below.

-   -   External Interruption: The transaction abort code is set to 2,         and the condition code in the transaction abort PSW is set to 2.         The transaction abort PSW is stored as the external old PSW as a         part of external interruption processing.     -   Program Interruption (Unfiltered): A program interruption         condition that results in an interruption (that is, an         unfiltered condition) causes the transaction to be aborted with         code 4. The condition code in the transaction abort PSW is set         specific to the program interruption code. The transaction abort         PSW is stored as the program old PSW as a part of program         interruption processing.     -   An instruction that would otherwise result in a transaction         being aborted due to an operation exception may yield alternate         results: for a non-constrained transaction, the transaction may         instead abort with abort code 11 (restricted instruction); for a         constrained transaction, a transaction constraint program         interruption may be recognized instead of the operation         exception.     -   When a PER (Program Event Recording) event is recognized in         conjunction with any other unfiltered program exception         condition, the condition code is set to 3.     -   Machine Check Interruption: The transaction abort code is set to         5, and the condition code in the transaction abort PSW is set         to 2. The transaction abort PSW is stored as the machine check         old PSW as a part of machine check interruption processing.     -   I/O Interruption: The transaction abort code is set to 6, and         the condition code in the transaction abort PSW is set to 2. The         transaction abort PSW is stored as the I/O old PSW as a part of         I/O interruption processing.     -   Fetch Overflow: A fetch overflow condition is detected when the         transaction attempts to fetch from more locations than the CPU         supports. The transaction abort code is set to 7, and the         condition code is set to either 2 or 3.     -   Store Overflow: A store overflow condition is detected when the         transaction attempts to store to more locations than the CPU         supports. The transaction abort code is set to 8, and the         condition code is set to either 2 or 3.     -   Allowing the condition code to be either 2 or 3 in response to a         fetch or store overflow abort allows the CPU to indicate         potentially retryable situations (e.g., condition code 2         indicates re-execution of the transaction may be productive;         while condition code 3 does not recommend re-execution).     -   Fetch Conflict: A fetch conflict condition is detected when         another CPU or the I/O subsystem attempts to store into a         location that has been transactionally fetched by this CPU. The         transaction abort code is set to 9, and the condition code is         set to 2.     -   Store Conflict: A store conflict condition is detected when         another CPU or the I/O subsystem attempts to access a location         that has been stored during transactional execution by this CPU.         The transaction abort code is set to 10, and the condition code         is set to 2.     -   Restricted Instruction: When the CPU is in the transactional         execution mode, attempted execution of a restricted instruction         causes the transaction to be aborted. The transaction abort code         is set to 11, and the condition code is set to 3.     -   When the CPU is in the constrained transactional execution mode,         it is unpredictable whether attempted execution of a restricted         instruction results in a transaction constraint program         interruption or an abort due to a restricted instruction. The         transaction is still aborted but the abort code may indicate         either cause.     -   Program Exception Condition (Filtered): A program exception         condition that does not result in an interruption (that is, a         filtered condition) causes the transaction to be aborted with a         transaction abort code of 12. The condition code is set to 3.     -   Nesting Depth Exceeded: The nesting depth exceeded condition is         detected when the transaction nesting depth is at the maximum         allowable value for the configuration, and a TRANSACTION BEGIN         instruction is executed. The transaction is aborted with a         transaction abort code of 13, and the condition code is set to         3.     -   Cache Fetch Related Condition: A condition related to storage         locations fetched by the transaction is detected by the CPU's         cache circuitry. The transaction is aborted with a transaction         abort code of 14, and the condition code is set to either 2 or         3.     -   Cache Store Related Condition: A condition related to storage         locations stored by the transaction is detected by the CPU's         cache circuitry. The transaction is aborted with a transaction         abort code of 15, and the condition code is set to either 2 or         3.     -   Cache Other Condition: A cache other condition is detected by         the CPU's cache circuitry. The transaction is aborted with a         transaction abort code of 16, and the condition code is set to         either 2 or 3.     -   During transactional execution, if the CPU accesses instructions         or storage operands using different logical addresses that are         mapped to the same absolute address, it is model dependent         whether the transaction is aborted. If the transaction is         aborted due to accesses using different logical addresses mapped         to the same absolute address, abort code 14, 15, or 16 is set,         depending on the condition.     -   CTEND Negative Operand Condition: A CTEND negative operand         condition is recognized when the CONDITIONAL TRANSACTION END         instruction is executed, and the contents of the second operand         are negative. The transaction is aborted with a transaction         abort code of 17, and the condition code is set to 3.     -   CTEND Time Out Condition: A CTEND time out condition is         recognized when the execution of a CONDITIONAL TRANSACTION END         instruction exceeds a model-dependent limit. The transaction         abort code is set to 18, and the condition code is set to 2.     -   Miscellaneous Condition: A miscellaneous condition is any other         condition recognized by the CPU that causes the transaction to         abort. The transaction abort code is set to 255 and the         condition code is set to either 2 or 3.     -   When multiple configurations are executing in the same machine         (for example, logical partitions or virtual machines), a         transaction may be aborted due to an external machine check or         I/O interruption that occurred in a different configuration.     -   Although examples are provided above, other causes of a         transaction abort with corresponding abort codes and condition         codes may be provided. For instance, a cause may be a Restart         Interruption, in which the transaction abort code is set to 1,         and the condition code in the transaction abort PSW is set to 2.         The transaction abort PSW is stored as the restart-old PSW as a         part of restart processing. As a further example, a cause may be         a Supervisor Call condition, in which the abort code is set to         3, and the condition code in the transaction abort PSW is set         to 3. Other or different examples are also possible.

Notes:

-   -   1. The miscellaneous condition may result from any of the         following:         -   Instructions, such as, in the z/Architecture, COMPARE AND             REPLACE DAT TABLE ENTRY, COMPARE AND SWAP AND PURGE,             INVALIDATE DAT TABLE ENTRY, INVALIDATE PAGE TABLE ENTRY,             PERFORM FRAME MANAGEMENT FUNCTION in which the NQ control is             zero and the SK control is one, SET STORAGE KEY EXTENDED in             which the NQ control is zero, performed by another CPU in             the configuration; the condition code is set to 2.         -   An operator function, such as reset, restart or stop, or the             equivalent SIGNAL PROCESSOR order is performed on the CPU.             The condition code is set to 2.         -   Any other condition not enumerated above; the condition code             is set to 2 or 3.     -   2. The location at which fetch and store conflicts are detected         may be anywhere within the same cache line.     -   3. Under certain conditions, the CPU may not be able to         distinguish between similar abort conditions. For example, a         fetch or store overflow may be indistinguishable from a         respective fetch or store conflict.     -   4. Speculative execution of multiple instruction paths by the         CPU may result in a transaction being aborted due to conflict or         overflow conditions, even if such conditions do not occur in the         conceptual sequence. While in the constrained transactional         execution mode, the CPU may temporarily inhibit speculative         execution, allowing the transaction to attempt to complete         without detecting such conflicts or overflows speculatively.

Execution of a TRANSACTION ABORT instruction causes the transaction to abort. The transaction abort code is set from the second operand address. The condition code is set to either 2 or 3, depending on whether bit 63 of the second operand address is zero or one, respectively.

Those skilled in the art understand that transaction execution advantageously improves parallel programming. Transactional processing advantageously simplifies the programmer's task by avoiding the need to manage locks. The multi-processor transactional execution environment, however, employs hardware which utilizes simple resolution of conflicts. For instance, one transaction may need to be aborted and the other rolled back. One transaction may be selected using a static selection approach. In one or more designs, this may be the transaction (i.e., local transaction) on the system receiving a request (i.e., requestor transaction) in an interference.

One transaction is selected over the other transaction because it enables the current protocol on the cache to be used. A request is received, response sent, and there are no changes to the protocol. However, the static selection approach of preferring, for instance, a requestor transaction over a local transaction, could disadvantageously impact performance. For instance, assume that a transaction has processed, for instance, 100,000 instructions, and that the requestor transaction is a newly started transaction, and is sending a request with, for instance, only one instruction executed so far. If the local transaction is aborted, and the instruction with the 100,000 instructions already executed is the local transaction, then those 100,000 instructions are aborted, and must be re-executed in order to allow the newly started, requestor transaction to proceed.

In accordance with one or more embodiments of the concepts disclosed herein, at least one protocol action is augmented with a metric associated with that request. When the protocol request is received, and a protocol request is found to interfere with a transaction on, for instance, a current or local processor, comparison of the execution progress of the respective transactions may be employed. Based on the comparison, either the requestor protocol request is rejected with an indication to roll back the requestor transaction, or, for instance, the local transaction is aborted, and the protocol request is satisfied. The “winner” can be chosen based on having the higher or lower metric, in accordance with a definition of the metric. In one or more implementations disclosed herein, the execution progress is a metric of work performed for the transaction. For instance, the metric may comprise at least one of transaction instructions processed or cycles elapsed for the transaction. The metric may represent an “earned value”, which is a dynamically changing value corresponding to the amount of work a particular transaction has performed. When two transactions interfere, one transaction must be aborted. Therefore, the metric may be employed, in one or more embodiments, to facilitate aborting the transaction that has only more recently been started, over the one that has already been executing. Incrementing of the metric may be performed by counting, for instance, the number of instructions processed or cycles elapsed for the transaction. The counting of, for instance, processed instructions, may be, in one or more implementations, a counting of completed instructions, committed instructions, decoded instructions, etc., of the transaction. In addition to counters for tracking the metric, a protocol request packet is provided which includes the execution progress of the requestor transaction. The protocol request is operatively coupled to logic for comparing the execution progresses, should there be an interference. The protocol response may direct a requestor transaction to abort, which may be operatively coupled to logic to abort a requestor transaction when such a response is received.

In one or more implementations, the above-discussed access logic may be combined with one or more other decisions, which may, for instance, involve priorities that are statically assigned or determined. For instance, one of the transactions may be characterized as a super-transaction, which means that that particular transaction wins when there is a conflict with another transaction, for instance, of another thread, within the multi-processor transactional execution environment. When neither of the transactions is a super-transaction, a computer system assigned conflict priority may be considered in determining if one of the transactions in the interference has a higher priority. If so, then that higher conflict priority may dictate which transaction continues and which aborts. If neither of the transactions is a super-transaction, or has a higher conflict priority than the other, then the transaction to continue may be selected using the execution progress concepts disclosed herein.

Generally stated, in one or more implementations, a method of accessing at least one memory location by one of a plurality of transactions in a multi-processor transactional execution environment is described. The method includes: tracking execution progress of a transaction, the execution progress being a metric of work performed for the transaction. For instance, the metric may be a count of at least one of instructions processed or cycles elapsed for the transaction since beginning execution of the transaction. In particular, the metric of work performed may include at least one of transaction instructions executed since a TBEGIN instruction, or cycles elapsed since a TBEGIN instruction. Alternatively, it may include a ratio of, for example, a total number of instructions performed over a total number of instructions predicted (e.g., by a hardware predictor, or a count in the TBEGIN instruction, or by another means). As one specific example, the metric of work performed for the transaction may be incremented based on transaction instructions processed or cycles elapsed in a decode stream for the corresponding transaction.

Based on encountering a conflict with another process for a memory location, the method may include comparing the execution progress of the transaction and the execution progress of the other the other process, and deciding whether to continue the transaction based on the comparing of the execution progress of the transaction and the execution progress of the other the other process.

In one or more implementations, the deciding may include: based on the execution progress of the transaction being greater than the execution progress of the other the other process, continuing the transaction, and based on the execution progress of the transaction being less than the execution progress of the other the other process, aborting the transaction. In one or more embodiments, the other the other process may be another transaction. Also, as noted, the other the other transaction may send the transaction a protocol request packet which includes the transaction progress of the other the other transaction.

Based on the comparison, either the requestor's protocol request is rejected with an indication to roll back the remote transaction, or the transaction (e.g., local transaction) is aborted, and the protocol request is satisfied.

As mentioned, in one or more implementations, a combination of decisions may be utilized in selecting which transaction to continue. For instance, one or more priority designations may be employed. A super-transaction may be defined as a transaction which always wins in an interference, with the super-transaction being assigned, for instance, by executing an instruction to designate transactions of a thread (thread-based) or a specific transaction of a thread (transaction-based), as a super-transaction. In at least one embodiment, there may also, or alternatively, be provided an instruction to set the priority of a transaction when multiple priorities are supported. In another embodiment, a TBEGIN instruction may be equipped to designate a specific transaction of a thread, as a super-transaction. In at least one embodiment, there may also be provided a TBEGIN instruction to set the transaction priority to one of several possible values when multiple priorities are supported. In yet another embodiment, a system call or library call may be performed to designate a specific transaction, for instance, of a thread, as a super-transaction. In at least one embodiment, there may also be a system call or library call which may be performed to set a transaction's priority to one of several possible values when multiple priorities are supported. In yet another embodiment, circuitry, firmware logic, supervisory software logic, and/or application logic may be provided to designate a specific transaction as a super-transaction, for instance, responsive to a transaction having been repeatedly aborted, to ensure application progress and equitable allocation of resources between a plurality of threads and processes implementing transaction execution. In one or more embodiments, there may also be provided hardware logic, firmware logic, supervisor program logic, or application logic to set a transaction's priority to one of several possible values when multiple priorities are supported (for instance, responsive to a transaction having been repeatedly aborted) to ensure application progress and equitable allocation of resources between a plurality of threads and processes implementing transaction execution. When neither of the transactions is a super-transaction, then an assigned conflict priority may be compared, that is, assuming that the transactions have conflict priorities associated therewith. If the transactions have the same priority, and neither is a super-transaction, then the transaction to continue may be selected using execution progress based on the comparison described herein.

As noted, a protocol request packet may be generated responsive to a memory access instruction to be performed under transaction execution. A requesting processor or CPU may format, for instance, a request field, to include an indication that the processor requests data being used by a transaction on another processor. A tag field may be used by future read/response exchanges to identify this exchange. For example, when a CPU or processor has multiple requests outstanding, a field such as the tag field may allow the requesting processor to more easily associate the responses with the request. Further, the request may include a field configured to indicate a type of access requested to the data, such as an exclusive access, where the requesting processor may intend to alter the data, or a shared access, where the requesting processor may only intend to read the data. An address field may specify the memory location of data for which access is being requested. In one or more implementations, a priority field may be provided, configured to indicate a conflict priority, or whether the transaction is a super-transaction that is making the data access request.

In an exemplary response packet, where the requesting processor or transactions successfully prevails in the interference evaluation, the protocol request field may contain an indication, such as a key word or an instruction (i.e., READRESPONSE) that the response packet contains the requested data in a data field. Where the requesting transaction is unsuccessful, the response packet may contain, for instance, in the protocol request field, an indication, such as a key word or an instruction (i.e., READREJECT), that the response packet does not contain the requested data in the data field. Those skilled in the art will understand that, in at least one implementation, the key word may be a defined sequence of one or more binary digits (bits) of information.

In one or more further embodiments, the priority information for a transaction may be provided with the instruction, for instance, the operand of the instruction may include a designation whether the conflict priority is “high”, “medium”, or “low”. In a further implementation, the operand could designate whether the instruction is associated with a super-transaction.

In one or more embodiments, at the beginning of a transaction, a conflict priority may be assigned to the transaction. In one or more implementations, setting a priority may include executing, by a computer system, a conflict priority setting instruction, which may include a mnemonic field having a mnemonic value, and an operand corresponding to the conflict priority. A transaction may be started immediately with an assigned conflict priority.

By way of specific example, FIGS. 8A-8C depict exemplary embodiments of logic for controlling access to a memory location within a multi-processor transactional execution environment, in accordance with one or more aspects of the present invention.

Referring initially to FIG. 8A, with beginning processing of a transaction 800, the computer system may assign 805 a priority designation to the transaction, such as super-transaction, or a conflict priority, such as “high”, “medium”, or “low”, as discussed above. Execution progress of the transaction is tracked 810. This may involve tracking a metric of work performed for the transaction, and in one or more implementations, include tracking at least one of transaction instructions processed or cycles elapsed, as noted above. In one embodiment, the tracking of execution progress may involve tracking a fraction of the overall execution of a transaction, e.g., the number of instructions that have been fetched for the subject transaction. In one embodiment, a prediction may be performed using a stored last one of a transaction size of the subject transaction. In another embodiment, the prediction may correspond to one of a minimum, maximum, average, median, mean, or another arithmetic function of two or more of several preceding executions of the transaction. During processing of the transaction, a request, such as a cross-interrogate (XI) request, is received for data, along with an associated execution progress of the requestor process requesting the data 815. Upon receipt of the request, an evaluation is made whether a conflict has been encountered for a memory location 820. If “no”, then logic determines whether the end of the transaction instructions has been reached 825, and if not, continues to track execution progress of the transaction. If “yes”, then the transaction results are committed 830.

FIG. 8B presents one embodiment of logic for processing a detected conflict for a memory location. As illustrated, logic initially determines whether the transaction or the requestor process (e.g., another transaction) has, for instance, the greater associated metric 835. If the requestor process has a higher metric representative of its execution progress, then the transaction (e.g., the local transaction) is aborted 840, and the requestor process is provided with the requested data 845. In addition, transaction failure processing is initiated for the (local) transaction 850.

Assuming that the transaction has the higher associated metric, then the request for data is rejected 855, and the logic returns to the flow of FIG. 8A. In such a case, the requestor process (e.g., another transaction making the request) receives the rejection 860, which aborts the requestor process 826, as well as initiates (transaction) failure processing 864, for instance, at the other the other processor processing the requestor process.

FIG. 8C depicts another embodiment of logic for processing a detected conflict for a memory location, which as explained below, may be used in combination with the logic of FIGS. 8A & 8B. For instance, rather than proceed directly to the logic of FIG. 8B, then from detection of a conflict at step 820 in FIG. 8A, logic may proceed to FIG. 8C, to determine whether either the transaction or the other the other process (e.g., another transaction) is a super-transaction 870. If one of the transactions is designated the super-transaction, then as noted above, that transaction wins in an interference. Therefore, the super-transaction is allowed to continue, and the other process or transaction is aborted 875, before returning to the logic flow of FIG. 8A to determine whether the end of the transaction instructions has been reached.

Assuming that neither transaction is a super-transaction, then, in one or more implementations, the logic compares assigned conflict priorities to determine if one is higher than the other 880. As noted above, in one or more implementations, a conflict priority may be assigned to a transaction statically by the computer system upon beginning the particular transaction. These conflict priorities may indicate a relative importance of the transaction relative to other transactions in the multi-processor transactional execution environment. For instance, the designated conflict priority for the transaction may be one of “high”, “medium”, or “low”. Thus, inquiry 880 in FIG. 8C determines whether one assigned conflict priority is higher than the other. If so, then the transaction with the higher priority is continued, and the other is aborted 885, and processing returns to the logic flow of FIG. 8A.

Assuming that neither transaction is a super-transaction, and that there is either no assigned conflict priorities or, if there are assigned conflict priorities, that they are at an equal level, then processing may pass to the logic of FIG. 8B to determine which transaction has the greater associated metric as a means of resolving the conflict, as described above.

FIGS. 9A & 9B depict a further embodiment of logic for controlling access to at least one memory location by one of a plurality of transactions in a multi-processor transactional execution environment.

Referring to FIG. 9A, processing starts 900 with optionally statically-assigning by the system a priority designation to a transaction 905. The priority designation may comprise, for instance, designating the transaction as the super-transaction, or alternatively, generating by the computer system a conflict priority for the transaction, which may be one of multiple pre-defined priority levels, such as “high”, “medium”, and “low”.

In accordance with one or more aspects of the present invention, execution progress is dynamically tracked for the transaction 910. The execution progress may be a metric of work performed for the transaction, and may include at least one of transaction instructions processed or cycles elapsed. For instance, the tracking may include incrementing a counter based on instructions processed or cycles elapsed in a decode stream for the transaction 915. Alternatively, or additionally, tracking execution progress may include tracking at least one of transaction instructions executed since a TBEGIN instruction, or cycles elapsed since the TBEGIN instruction 920.

Based on encountering a conflict with another process (e.g., another transaction), for a memory location 925, one or more of the following actions may be undertaken.

For instance, the logic may determine whether the transaction or the other the other process (e.g., another transaction) is a super-transaction, and based on the transaction or the other the other transaction being the super-transaction, continue the super-transaction and refrain from comparing execution progress of the transaction and the other the other transaction 930.

Still further, logic may determine whether the transaction or the other the other transaction has a higher assigned conflict priority, and based on one of the transaction or the other the other transaction having higher assigned conflict priority, continue that transaction and refrain from comparing execution progress of the transaction and the other the other transaction 935.

Still further, logic may compare the execution progress of the transaction and the other the other process (e.g., another transaction) 940 (FIG. 9B), which may have been received in a protocol request packet associated with the other the other transaction. Logic then decides whether to continue the transaction based on the comparing of the execution progress of the transaction and the execution progress of the other the other process 945. This action may include, for instance: based on the execution progress of the transaction being greater than the execution progress of the other the other process, continuing the transaction, and based on the execution progress of the transaction being less than the execution progress of the other the other process, aborting the transaction; or based on the execution progress of the transaction being greater than the execution progress of the other the other process (e.g., another transaction), sending a response to the other the other transaction rejecting access to the memory location, and based on the execution progress of the transaction being less than the execution progress of the other the other transaction, sending a response to the other the other transaction comprising data corresponding to the memory location 955. This completes 960 the exemplary logic embodiment of FIGS. 9A & 9B.

In one example, a computer program product may include, for instance, one or more non-transitory computer readable storage media to store computer readable program code means, logic and/or instructions thereon to provide and facilitate one or more embodiments.

The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.

The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

In addition to the above, one or more aspects may be provided, offered, deployed, managed, serviced, etc. by a service provider who offers management of customer environments. For instance, the service provider can create, maintain, support, etc. computer code and/or a computer infrastructure that performs one or more aspects for one or more customers. In return, the service provider may receive payment from the customer under a subscription and/or fee agreement, as examples. Additionally or alternatively, the service provider may receive payment from the sale of advertising content to one or more third parties.

In one aspect, an application may be deployed for performing one or more embodiments. As one example, the deploying of an application comprises providing computer infrastructure operable to perform one or more embodiments.

As a further aspect, a computing infrastructure may be deployed comprising integrating computer readable code into a computing system, in which the code in combination with the computing system is capable of performing one or more embodiments.

As yet a further aspect, a process for integrating computing infrastructure comprising integrating computer readable code into a computer system may be provided. The computer system comprises a computer readable medium, in which the computer medium comprises one or more embodiments. The code in combination with the computer system is capable of performing one or more embodiments.

Although various embodiments are described above, these are only examples. For example, computing environments of other architectures can be used to incorporate and use one or more embodiments. Further, different instructions, instruction formats, instruction fields and/or instruction values may be used. Yet further, although examples of values for abort codes and condition codes are provided, other values may be used. Moreover, different, other, and/or additional restrictions/constraints may be provided/used. Yet further, other intervals may be provided and/or used in differing ways. Many variations are possible.

Further, other types of computing environments can benefit and be used. As an example, a data processing system suitable for storing and/or executing program code is usable that includes at least two processors coupled directly or indirectly to memory elements through a system bus. The memory elements include, for instance, local memory employed during actual execution of the program code, bulk storage, and cache memory which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution.

Input/Output or I/O devices (including, but not limited to, keyboards, displays, pointing devices, DASD, tape, CDs, DVDs, thumb drives and other memory media, etc.) can be coupled to the system either directly or through intervening I/O controllers. Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modems, and Ethernet cards are just a few of the available types of network adapters.

Another embodiment of a computing environment to incorporate and use one or more aspects is described with reference to FIG. 10A. In this example, a computing environment 4000 includes, for instance, a native central processing unit 4002, a memory 4004, and one or more input/output devices and/or interfaces 4006 coupled to one another via, for example, one or more buses 4008 and/or other connections. As examples, computing environment 40G00 may include a PowerPC processor, a zSeries server, or a pSeries server offered by International Business Machines Corporation, Armonk, N.Y.; an HP Superdome with Intel Itanium II processors offered by Hewlett Packard Co., Palo Alto, Calif.; and/or other machines based on architectures offered by International Business Machines Corporation, Hewlett Packard, Intel, Oracle, or others.

Native central processing unit 4002 includes one or more native registers 4010, such as one or more general purpose registers and/or one or more special purpose registers used during processing within the environment. These registers include information that represents the state of the environment at any particular point in time.

Moreover, native central processing unit 4002 executes instructions and code that are stored in memory 4004. In one particular example, the central processing unit executes emulator code 4012 stored in memory 4004. This code enables the processing environment configured in one architecture to emulate another architecture. For instance, emulator code 4012 allows machines based on architectures other than the Power architecture, such as zSeries servers, pSeries servers, HP Superdome servers or others, to emulate the Power architecture and to execute software and instructions developed based on the Power architecture. In a further example, emulator code 4012 allows machines based on architectures other than the z/Architecture, such as PowerPC processors, pSeries servers, HP Superdome servers or others, to emulate the z/Architecture and to execute software and instructions developed based on the z/Architecture. Other architectures may also be emulated.

Further details relating to emulator code 4012 are described with reference to FIG. 10B. Guest instructions 4050 stored in memory 4004 comprise software instructions (e.g., correlating to machine instructions) that were developed to be executed in an architecture other than that of native CPU 4002. For example, guest instructions 4050 may have been designed to execute on a PowerPC processor or a z/Architecture processor, but instead, are being emulated on native CPU 4002, which may be, for example, an Intel Itanium II processor. In one example, emulator code 4012 includes an instruction fetching routine 4052 to obtain one or more guest instructions 4050 from memory 4004, and to optionally provide local buffering for the instructions obtained. It also includes an instruction translation routine 4054 to determine the type of guest instruction that has been obtained and to translate the guest instruction into one or more corresponding native instructions 4056. This translation includes, for instance, identifying the function to be performed by the guest instruction and choosing the native instruction(s) to perform that function.

Further, emulator code 4012 includes an emulation control routine 4060 to cause the native instructions to be executed. Emulation control routine 4060 may cause native CPU 4002 to execute a routine of native instructions that emulate one or more previously obtained guest instructions and, at the conclusion of such execution, return control to the instruction fetch routine to emulate the obtaining of the next guest instruction or a group of guest instructions. Execution of the native instructions 4056 may include loading data into a register from memory 4004; storing data back to memory from a register; or performing some type of arithmetic or logic operation, as determined by the translation routine.

Each routine is, for instance, implemented in software, which is stored in memory and executed by native central processing unit 4002. In other examples, one or more of the routines or operations are implemented in firmware, hardware, software or some combination thereof. The registers of the emulated processor may be emulated using registers 4010 of the native CPU or by using locations in memory 4004. In embodiments, guest instructions 4050, native instructions 4056 and emulator code 4012 may reside in the same memory or may be disbursed among different memory devices.

As used herein, firmware includes, e.g., the microcode, millicode and/or macrocode of the processor. It includes, for instance, the hardware-level instructions and/or data structures used in implementation of higher level machine code. In one embodiment, it includes, for instance, proprietary code that is typically delivered as microcode that includes trusted software or microcode specific to the underlying hardware and controls operating system access to the system hardware.

Referring to FIG. 11, representative components of a Host Computer system 5000 to implement one or more embodiments are portrayed. The representative host computer 5000 comprises one or more CPUs 5001 in communication with computer memory (i.e., central storage) 5002, as well as I/O interfaces to storage media devices 5011 and networks 5010 for communicating with other computers or SANs and the like. The CPU 5001 is compliant with an architecture having an architected instruction set and architected functionality. The CPU 5001 may have access register translation (ART) 5012, which includes an ART lookaside buffer (ALB) 5013, for selecting an address space to be used by dynamic address translation (DAT) 5003 for transforming program addresses (virtual addresses) into real addresses of memory. A DAT typically includes a translation lookaside buffer (TLB) 5007 for caching translations so that later accesses to the block of computer memory 5002 do not require the delay of address translation. Typically, a cache 5009 is employed between computer memory 5002 and the processor 5001. The cache 5009 may be hierarchical having a large cache available to more than one CPU and smaller, faster (lower level) caches between the large cache and each CPU. In some implementations, the lower level caches are split to provide separate low level caches for instruction fetching and data accesses. In one embodiment, for the TX facility, a transaction diagnostic block (TDB) 5100 and one or more buffers 5101 may be stored in one or more of cache 5009 and memory 5002. In one example, in TX mode, data is initially stored in a TX buffer, and when TX mode ends (e.g., outermost TEND), the data in the buffer is stored (committed) to memory, or if there is an abort, the data in the buffer is discarded.

In one embodiment, an instruction is fetched from memory 5002 by an instruction fetch unit 5004 via a cache 5009. The instruction is decoded in an instruction decode unit 5006 and dispatched (with other instructions in some embodiments) to instruction execution unit or units 5008. Typically several execution units 5008 are employed, for example an arithmetic execution unit, a floating point execution unit and a branch instruction execution unit. Further, in one embodiment of the TX facility, various TX controls 5110 may be employed. The instruction is executed by the execution unit, accessing operands from instruction specified registers or memory as needed. If an operand is to be accessed (loaded or stored) from memory 5002, a load/store unit 5005 typically handles the access under control of the instruction being executed. Instructions may be executed in hardware circuits or in internal microcode (firmware) or by a combination of both.

In accordance with an aspect of the TX facility, processor 5001 also includes a PSW 5102 (e.g., TX and/or abort PSW), a nesting depth 5104, a TDBA 5106, and one or more control registers 5108.

As noted, a computer system includes information in local (or main) storage, as well as addressing, protection, and reference and change recording. Some aspects of addressing include the format of addresses, the concept of address spaces, the various types of addresses, and the manner in which one type of address is translated to another type of address. Some of main storage includes permanently assigned storage locations. Main storage provides the system with directly addressable fast-access storage of data. Both data and programs are to be loaded into main storage (from input devices) before they can be processed.

Main storage may include one or more smaller, faster-access buffer storages, sometimes called caches. A cache is typically physically associated with a CPU or an I/O processor. The effects, except on performance, of the physical construction and use of distinct storage media are generally not observable by the program.

Separate caches may be maintained for instructions and for data operands. Information within a cache is maintained in contiguous bytes on an integral boundary called a cache block or cache line (or line, for short). A model may provide an EXTRACT CACHE ATTRIBUTE instruction which returns the size of a cache line in bytes. A model may also provide PREFETCH DATA and PREFETCH DATA RELATIVE LONG instructions which effects the prefetching of storage into the data or instruction cache or the releasing of data from the cache.

Storage is viewed as a long horizontal string of bits. For most operations, accesses to storage proceed in a left-to-right sequence. The string of bits is subdivided into units of eight bits. An eight-bit unit is called a byte, which is the basic building block of all information formats. Each byte location in storage is identified by a unique nonnegative integer, which is the address of that byte location or, simply, the byte address. Adjacent byte locations have consecutive addresses, starting with 0 on the left and proceeding in a left-to-right sequence. Addresses are unsigned binary integers and are 24, 31, or 64 bits.

Information is transmitted between storage and a CPU or a channel subsystem one byte, or a group of bytes, at a time. Unless otherwise specified, in, for instance, the z/Architecture, a group of bytes in storage is addressed by the leftmost byte of the group. The number of bytes in the group is either implied or explicitly specified by the operation to be performed. When used in a CPU operation, a group of bytes is called a field. Within each group of bytes, in, for instance, the z/Architecture, bits are numbered in a left-to-right sequence. In the z/Architecture, the leftmost bits are sometimes referred to as the “high-order” bits and the rightmost bits as the “low-order” bits. Bit numbers are not storage addresses, however. Only bytes can be addressed. To operate on individual bits of a byte in storage, the entire byte is accessed. The bits in a byte are numbered 0 through 7, from left to right (in, e.g., the z/Architecture). The bits in an address may be numbered 8-31 or 40-63 for 24-bit addresses, or 1-31 or 33-63 for 31-bit addresses; they are numbered 0-63 for 64-bit addresses. In one example, bits 8-31 and 1-31 apply to addresses that are in a location (e.g., register) that is 32 bits wide, whereas bits 40-63 and 33-63 apply to addresses that are in a 64-bit wide location. Within any other fixed-length format of multiple bytes, the bits making up the format are consecutively numbered starting from 0. For purposes of error detection, and in preferably for correction, one or more check bits may be transmitted with each byte or with a group of bytes. Such check bits are generated automatically by the machine and cannot be directly controlled by the program. Storage capacities are expressed in number of bytes. When the length of a storage-operand field is implied by the operation code of an instruction, the field is said to have a fixed length, which can be one, two, four, eight, or sixteen bytes. Larger fields may be implied for some instructions. When the length of a storage-operand field is not implied but is stated explicitly, the field is said to have a variable length. Variable-length operands can vary in length by increments of one byte (or with some instructions, in multiples of two bytes or other multiples). When information is placed in storage, the contents of only those byte locations are replaced that are included in the designated field, even though the width of the physical path to storage may be greater than the length of the field being stored.

Certain units of information are to be on an integral boundary in storage. A boundary is called integral for a unit of information when its storage address is a multiple of the length of the unit in bytes. Special names are given to fields of 2, 4, 8, 16, and 32 bytes on an integral boundary. A halfword is a group of two consecutive bytes on a two-byte boundary and is the basic building block of instructions. A word is a group of four consecutive bytes on a four-byte boundary. A doubleword is a group of eight consecutive bytes on an eight-byte boundary. A quadword is a group of 16 consecutive bytes on a 16-byte boundary. An octoword is a group of 32 consecutive bytes on a 32-byte boundary. When storage addresses designate halfwords, words, doublewords, quadwords, and octowords, the binary representation of the address contains one, two, three, four, or five rightmost zero bits, respectively. Instructions are to be on two-byte integral boundaries. The storage operands of most instructions do not have boundary-alignment requirements.

On devices that implement separate caches for instructions and data operands, a significant delay may be experienced if the program stores into a cache line from which instructions are subsequently fetched, regardless of whether the store alters the instructions that are subsequently fetched.

In one example, the embodiment may be practiced by software (sometimes referred to licensed internal code, firmware, micro-code, milli-code, pico-code and the like, any of which would be consistent with one or more embodiments). Referring to FIG. 16, software program code which embodies one or more aspects may be accessed by processor 5001 of the host system 5000 from long-term storage media devices 5011, such as a CD-ROM drive, tape drive or hard drive. The software program code may be embodied on any of a variety of known media for use with a data processing system, such as a diskette, hard drive, or CD-ROM. The code may be distributed on such media, or may be distributed to users from computer memory or central storage 5002, or storage of one computer system over a network 5010 to other computer systems for use by users of such other systems.

The software program code includes an operating system which controls the function and interaction of the various computer components and one or more application programs. Program code is normally paged from storage media device 5011 to the relatively higher-speed computer storage 5002 where it is available for processing by processor 5001. The techniques and methods for embodying software program code in memory, on physical media, and/or distributing software code via networks are well known and will not be further discussed herein. Program code, when created and stored on a tangible medium (including but not limited to electronic memory modules (RAM), flash memory, Compact Discs (CDs), DVDs, Magnetic Tape and the like is often referred to as a “computer program product”. The computer program product medium is typically readable by a processing circuit preferably in a computer system for execution by the processing circuit.

The cache that is most readily available to the processor (normally faster and smaller than other caches of the processor) is the lowest (L1 or level one) cache and main store (main memory) is the highest level cache (L3 if there are 3 levels). The lowest level cache is often divided into an instruction cache (I-Cache) holding machine instructions to be executed and a data cache (D-Cache) holding data operands.

Referring to FIG. 12, an exemplary processor embodiment is depicted for processor 5026. Typically one or more levels of cache 5053 are employed to buffer memory blocks in order to improve processor performance. The cache 5053 is a high speed buffer holding cache lines of memory data that are likely to be used. Typical cache lines are 64, 128 or 256 bytes of memory data. Separate caches are often employed for caching instructions than for caching data. Cache coherence (synchronization of copies of lines in memory and the caches) is often provided by various “snoop” algorithms well known in the art. Main memory storage 5025 of a processor system is often referred to as a cache. In a processor system having 4 levels of cache 5053, main storage 5025 is sometimes referred to as the level 5 (L5) cache since it is typically faster and only holds a portion of the non-volatile storage (DASD, tape etc.) that is available to a computer system. Main storage 5025 “caches” pages of data paged in and out of the main storage 5025 by the operating system.

A program counter (instruction counter) 5061 keeps track of the address of the current instruction to be executed. A program counter in a z/Architecture processor is 64 bits and can be truncated to 31 or 24 bits to support prior addressing limits. A program counter is typically embodied in a PSW (program status word) of a computer such that it persists during context switching. Thus, a program in progress, having a program counter value, may be interrupted by, for example, the operating system (context switch from the program environment to the operating system environment). The PSW of the program maintains the program counter value while the program is not active, and the program counter (in the PSW) of the operating system is used while the operating system is executing. Typically, the program counter is incremented by an amount equal to the number of bytes of the current instruction. RISC (Reduced Instruction Set Computing) instructions are typically fixed length while CISC (Complex Instruction Set Computing) instructions are typically variable length. Instructions of the IBM z/Architecture are CISC instructions having a length of 2, 4 or 6 bytes. The Program counter 5061 is modified by either a context switch operation or a branch taken operation of a branch instruction for example. In a context switch operation, the current program counter value is saved in the program status word along with other state information about the program being executed (such as condition codes), and a new program counter value is loaded pointing to an instruction of a new program module to be executed. A branch taken operation is performed in order to permit the program to make decisions or loop within the program by loading the result of the branch instruction into the program counter 5061.

Typically an instruction fetch unit 5055 is employed to fetch instructions on behalf of the processor 5026. The fetch unit either fetches “next sequential instructions”, target instructions of branch taken instructions, or first instructions of a program following a context switch. Modern Instruction fetch units often employ prefetch techniques to speculatively prefetch instructions based on the likelihood that the prefetched instructions might be used. For example, a fetch unit may fetch 16 bytes of instruction that includes the next sequential instruction and additional bytes of further sequential instructions.

The fetched instructions are then executed by the processor 5026. In an embodiment, the fetched instruction(s) are passed to a dispatch unit 5056 of the fetch unit. The dispatch unit decodes the instruction(s) and forwards information about the decoded instruction(s) to appropriate units 5057, 5058, 5060. An execution unit 5057 will typically receive information about decoded arithmetic instructions from the instruction fetch unit 5055 and will perform arithmetic operations on operands according to the opcode of the instruction. Operands are provided to the execution unit 5057 preferably either from memory 5025, architected registers 5059 or from an immediate field of the instruction being executed. Results of the execution, when stored, are stored either in memory 5025, registers 5059 or in other machine hardware (such as control registers, PSW registers and the like).

Virtual addresses are transformed into real addresses using dynamic address translation 5062 and, optionally, using access register translation 5063.

A processor 5026 typically has one or more units 5057, 5058, 5060 for executing the function of the instruction. Referring to FIG. 13A, an execution unit 5057 may communicate 5071 with architected general registers 5059, a decode/dispatch unit 5056, a load store unit 5060, and other 5065 processor units by way of interfacing logic 5071. An execution unit 5057 may employ several register circuits 5067, 5068, 5069 to hold information that the arithmetic logic unit (ALU) 5066 will operate on. The ALU performs arithmetic operations such as add, subtract, multiply and divide as well as logical function such as and, or and exclusive-or (XOR), rotate and shift. Preferably the ALU supports specialized operations that are design dependent. Other circuits may provide other architected facilities 5072 including condition codes and recovery support logic for example. Typically the result of an ALU operation is held in an output register circuit 5070 which can forward the result to a variety of other processing functions. There are many arrangements of processor units, the present description is only intended to provide a representative understanding of one embodiment.

An ADD instruction for example would be executed in an execution unit 5057 having arithmetic and logical functionality while a floating point instruction for example would be executed in a floating point execution having specialized floating point capability. Preferably, an execution unit operates on operands identified by an instruction by performing an opcode defined function on the operands. For example, an ADD instruction may be executed by an execution unit 5057 on operands found in two registers 5059 identified by register fields of the instruction.

The execution unit 5057 performs the arithmetic addition on two operands and stores the result in a third operand where the third operand may be a third register or one of the two source registers. The execution unit preferably utilizes an Arithmetic Logic Unit (ALU) 5066 that is capable of performing a variety of logical functions such as Shift, Rotate, And, Or and XOR as well as a variety of algebraic functions including any of add, subtract, multiply, divide. Some ALUs 5066 are designed for scalar operations and some for floating point. Data may be Big Endian (where the least significant byte is at the highest byte address) or Little Endian (where the least significant byte is at the lowest byte address) depending on architecture. The IBM z/Architecture is Big Endian. Signed fields may be sign and magnitude, 1's complement or 2's complement depending on architecture. A 2's complement number is advantageous in that the ALU does not need to design a subtract capability since either a negative value or a positive value in 2's complement requires only an addition within the ALU. Numbers are commonly described in shorthand, where a 12 bit field defines an address of a 4,096 byte block and is commonly described as a 4 Kbyte (Kilo-byte) block, for example.

Referring to FIG. 13B, branch instruction information for executing a branch instruction is typically sent to a branch unit 5058 which often employs a branch prediction algorithm such as a branch history table 5082 to predict the outcome of the branch before other conditional operations are complete. The target of the current branch instruction will be fetched and speculatively executed before the conditional operations are complete. When the conditional operations are completed the speculatively executed branch instructions are either completed or discarded based on the conditions of the conditional operation and the speculated outcome. A typical branch instruction may test condition codes and branch to a target address if the condition codes meet the branch requirement of the branch instruction, a target address may be calculated based on several numbers including ones found in register fields or an immediate field of the instruction for example. The branch unit 5058 may employ an ALU 5074 having a plurality of input register circuits 5075, 5076, 5077 and an output register circuit 5080. The branch unit 5058 may communicate 5081 with general registers 5059, decode dispatch unit 5056 or other circuits 5073, for example.

The execution of a group of instructions can be interrupted for a variety of reasons including a context switch initiated by an operating system, a program exception or error causing a context switch, an I/O interruption signal causing a context switch or multi-threading activity of a plurality of programs (in a multi-threaded environment), for example. Preferably a context switch action saves state information about a currently executing program and then loads state information about another program being invoked. State information may be saved in hardware registers or in memory for example. State information preferably comprises a program counter value pointing to a next instruction to be executed, condition codes, memory translation information and architected register content. A context switch activity can be exercised by hardware circuits, application programs, operating system programs or firmware code (microcode, pico-code or licensed internal code (LIC)) alone or in combination.

A processor accesses operands according to instruction defined methods. The instruction may provide an immediate operand using the value of a portion of the instruction, may provide one or more register fields explicitly pointing to either general purpose registers or special purpose registers (floating point registers for example). The instruction may utilize implied registers identified by an opcode field as operands. The instruction may utilize memory locations for operands. A memory location of an operand may be provided by a register, an immediate field, or a combination of registers and immediate field as exemplified by the z/Architecture long displacement facility wherein the instruction defines a base register, an index register and an immediate field (displacement field) that are added together to provide the address of the operand in memory for example. Location herein typically implies a location in main memory (main storage) unless otherwise indicated.

Referring to FIG. 13C, a processor accesses storage using a load/store unit 5060. The load/store unit 5060 may perform a load operation by obtaining the address of the target operand in memory 5053 and loading the operand in a register 5059 or another memory 5053 location, or may perform a store operation by obtaining the address of the target operand in memory 5053 and storing data obtained from a register 5059 or another memory 5053 location in the target operand location in memory 5053. The load/store unit 5060 may be speculative and may access memory in a sequence that is out-of-order relative to instruction sequence, however the load/store unit 5060 is to maintain the appearance to programs that instructions were executed in order. A load/store unit 5060 may communicate 5084 with general registers 5059, decode/dispatch unit 5056, cache/memory interface 5053 or other elements 5083 and comprises various register circuits 5086, 5087, 5088 and 5089, ALUs 5085 and control logic 5090 to calculate storage addresses and to provide pipeline sequencing to keep operations in-order. Some operations may be out of order but the load/store unit provides functionality to make the out of order operations to appear to the program as having been performed in order, as is well known in the art.

Preferably addresses that an application program “sees” are often referred to as virtual addresses. Virtual addresses are sometimes referred to as “logical addresses” and “effective addresses”. These virtual addresses are virtual in that they are redirected to physical memory location by one of a variety of dynamic address translation (DAT) technologies including, but not limited to, simply prefixing a virtual address with an offset value, translating the virtual address via one or more translation tables, the translation tables preferably comprising at least a segment table and a page table alone or in combination, preferably, the segment table having an entry pointing to the page table. In the z/Architecture, a hierarchy of translation is provided including a region first table, a region second table, a region third table, a segment table and an optional page table. The performance of the address translation is often improved by utilizing a translation lookaside buffer (TLB) which comprises entries mapping a virtual address to an associated physical memory location. The entries are created when the DAT translates a virtual address using the translation tables. Subsequent use of the virtual address can then utilize the entry of the fast TLB rather than the slow sequential translation table accesses. TLB content may be managed by a variety of replacement algorithms including LRU (Least Recently used).

In the case where the processor is a processor of a multi-processor system, each processor has responsibility to keep shared resources, such as I/O, caches, TLBs and memory, interlocked for coherency. Typically, “snoop” technologies will be utilized in maintaining cache coherency. In a snoop environment, each cache line may be marked as being in any one of a shared state, an exclusive state, a changed state, an invalid state and the like in order to facilitate sharing.

I/O units 5054 (FIG. 12) provide the processor with means for attaching to peripheral devices including tape, disc, printers, displays, and networks for example. I/O units are often presented to the computer program by software drivers. In mainframes, such as the System z from IBM®, channel adapters and open system adapters are I/O units of the mainframe that provide the communications between the operating system and peripheral devices.

In a further embodiment, one or more aspects relate to cloud computing. It is understood in advance that although this disclosure includes a detailed description on cloud computing, implementation of the teachings recited herein are not limited to a cloud computing environment. Rather, embodiments of the present invention are capable of being implemented in conjunction with any other type of computing environment now known or later developed.

Cloud computing is a model of service delivery for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, network bandwidth, servers, processing, memory, storage, applications, virtual machines, and services) that can be rapidly provisioned and released with minimal management effort or interaction with a provider of the service. This cloud model may include at least five characteristics, at least three service models, and at least four deployment models.

Characteristics are as follows:

On-demand self-service: a cloud consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with the service's provider.

Broad network access: capabilities are available over a network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, laptops, and PDAs).

Resource pooling: the provider's computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to demand. There is a sense of location independence in that the consumer generally has no control or knowledge over the exact location of the provided resources but may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter).

Rapid elasticity: capabilities can be rapidly and elastically provisioned, in some cases automatically, to quickly scale out and rapidly released to quickly scale in. To the consumer, the capabilities available for provisioning often appear to be unlimited and can be purchased in any quantity at any time.

Measured service: cloud systems automatically control and optimize resource use by leveraging a metering capability at some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Resource usage can be monitored, controlled, and reported providing transparency for both the provider and consumer of the utilized service.

Service Models are as follows:

Software as a Service (SaaS): the capability provided to the consumer is to use the provider's applications running on a cloud infrastructure. The applications are accessible from various client devices through a thin client interface such as a web browser (e.g., web-based email). The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user-specific application configuration settings.

Platform as a Service (PaaS): the capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or acquired applications created using programming languages and tools supported by the provider. The consumer does not manage or control the underlying cloud infrastructure including networks, servers, operating systems, or storage, but has control over the deployed applications and possibly application hosting environment configurations.

Infrastructure as a Service (IaaS): the capability provided to the consumer is to provision processing, storage, networks, and other fundamental computing resources where the consumer is able to deploy and run arbitrary software, which can include operating systems and applications. The consumer does not manage or control the underlying cloud infrastructure but has control over operating systems, storage, deployed applications, and possibly limited control of select networking components (e.g., host firewalls).

Deployment Models are as follows:

Private cloud: the cloud infrastructure is operated solely for an organization. It may be managed by the organization or a third party and may exist on-premises or off-premises.

Community cloud: the cloud infrastructure is shared by several organizations and supports a specific community that has shared concerns (e.g., mission, security requirements, policy, and compliance considerations). It may be managed by the organizations or a third party and may exist on-premises or off-premises.

Public cloud: the cloud infrastructure is made available to the general public or a large industry group and is owned by an organization selling cloud services.

Hybrid cloud: the cloud infrastructure is a composition of two or more clouds (private, community, or public) that remain unique entities but are bound together by standardized or proprietary technology that enables data and application portability (e.g., cloud bursting for loadbalancing between clouds).

A cloud computing environment is service oriented with a focus on statelessness, low coupling, modularity, and semantic interoperability. At the heart of cloud computing is an infrastructure comprising a network of interconnected nodes.

Referring now to FIG. 14, a schematic of an example of a cloud computing node is shown. Cloud computing node 6010 is only one example of a suitable cloud computing node and is not intended to suggest any limitation as to the scope of use or functionality of embodiments of the invention described herein. Regardless, cloud computing node 6010 is capable of being implemented and/or performing any of the functionality set forth hereinabove.

In cloud computing node 6010 there is a computer system/server 6012, which is operational with numerous other general purpose or special purpose computing system environments or configurations. Examples of well-known computing systems, environments, and/or configurations that may be suitable for use with computer system/server 6012 include, but are not limited to, personal computer systems, server computer systems, thin clients, thick clients, handheld or laptop devices, multi-processor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputer systems, mainframe computer systems, and distributed cloud computing environments that include any of the above systems or devices, and the like.

Computer system/server 6012 may be described in the general context of computer system executable instructions, such as program modules, being executed by a computer system. Generally, program modules may include routines, programs, objects, components, logic, data structures, and so on that perform particular tasks or implement particular abstract data types. Computer system/server 6012 may be practiced in distributed cloud computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed cloud computing environment, program modules may be located in both local and remote computer system storage media including memory storage devices.

As shown in FIG. 14, computer system/server 6012 in cloud computing node 6010 is shown in the form of a general-purpose computing device. The components of computer system/server 6012 may include, but are not limited to, one or more processors or processing units 6016, a system memory 6028, and a bus 6018 that couples various system components including system memory 6028 to processor 6016.

Bus 6018 represents one or more of any of several types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus.

Computer system/server 6012 typically includes a variety of computer system readable media. Such media may be any available media that is accessible by computer system/server 6012, and it includes both volatile and non-volatile media, removable and non-removable media.

System memory 6028 can include computer system readable media in the form of volatile memory, such as random access memory (RAM) 6030 and/or cache memory 6032. Computer system/server 6012 may further include other removable/non-removable, volatile/non-volatile computer system storage media. By way of example only, storage system 6034 can be provided for reading from and writing to a non-removable, non-volatile magnetic media (not shown and typically called a “hard drive”). Although not shown, a magnetic disk drive for reading from and writing to a removable, non-volatile magnetic disk (e.g., a “floppy disk”), and an optical disk drive for reading from or writing to a removable, non-volatile optical disk such as a CD-ROM, DVD-ROM or other optical media can be provided. In such instances, each can be connected to bus 6018 by one or more data media interfaces. As will be further depicted and described below, memory 6028 may include at least one program product having a set (e.g., at least one) of program modules that are configured to carry out the functions of embodiments of the invention.

Program/utility 6040, having a set (at least one) of program modules 6042, may be stored in memory 6028 by way of example, and not limitation, as well as an operating system, one or more application programs, other program modules, and program data. Each of the operating system, one or more application programs, other program modules, and program data or some combination thereof, may include an implementation of a networking environment. Program modules 6042 generally carry out the functions and/or methodologies of embodiments of the invention as described herein.

Computer system/server 6012 may also communicate with one or more external devices 6014 such as a keyboard, a pointing device, a display 6024, etc.; one or more devices that enable a user to interact with computer system/server 6012; and/or any devices (e.g., network card, modem, etc.) that enable computer system/server 6012 to communicate with one or more other computing devices. Such communication can occur via Input/Output (I/O) interfaces 6022. Still yet, computer system/server 6012 can communicate with one or more networks such as a local area network (LAN), a general wide area network (WAN), and/or a public network (e.g., the Internet) via network adapter 6020. As depicted, network adapter 6020 communicates with the other components of computer system/server 6012 via bus 6018. It should be understood that although not shown, other hardware and/or software components could be used in conjunction with computer system/server 6012. Examples, include, but are not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data archival storage systems, etc.

Referring now to FIG. 15, illustrative cloud computing environment 6050 is depicted. As shown, cloud computing environment 6050 comprises one or more cloud computing nodes 6010 with which local computing devices used by cloud consumers, such as, for example, personal digital assistant (PDA) or cellular telephone 6054A, desktop computer 6054B, laptop computer 6054C, and/or automobile computer system 6054N may communicate. Nodes 6010 may communicate with one another. They may be grouped (not shown) physically or virtually, in one or more networks, such as Private, Community, Public, or Hybrid clouds as described hereinabove, or a combination thereof. This allows cloud computing environment 6050 to offer infrastructure, platforms and/or software as services for which a cloud consumer does not need to maintain resources on a local computing device. It is understood that the types of computing devices 6054A-N shown in FIG. 15 are intended to be illustrative only and that computing nodes 6010 and cloud computing environment 6050 can communicate with any type of computerized device over any type of network and/or network addressable connection (e.g., using a web browser).

Referring now to FIG. 16, a set of functional abstraction layers provided by cloud computing environment 6050 (FIG. 14) is shown. It should be understood in advance that the components, layers, and functions shown in FIG. 16 are intended to be illustrative only and embodiments of the invention are not limited thereto. As depicted, the following layers and corresponding functions are provided:

Hardware and software layer 6060 includes hardware and software components. Examples of hardware components include mainframes, in one example IBM® zSeries® systems; RISC (Reduced Instruction Set Computer) architecture based servers, in one example IBM pSeries® systems; IBM xSeries® systems; IBM BladeCenter® systems; storage devices; networks and networking components. Examples of software components include network application server software, in one example IBM WebSphere® application server software; and database software, in one example IBM DB2® database software. (IBM, zSeries, pSeries, xSeries, BladeCenter, WebSphere, and DB2 are trademarks of International Business Machines Corporation registered in many jurisdictions worldwide).

Virtualization layer 6062 provides an abstraction layer from which the following examples of virtual entities may be provided: virtual servers; virtual storage; virtual networks, including virtual private networks; virtual applications and operating systems; and virtual clients.

In one example, management layer 6064 may provide the functions described below. Resource provisioning provides dynamic procurement of computing resources and other resources that are utilized to perform tasks within the cloud computing environment. Metering and Pricing provide cost tracking as resources are utilized within the cloud computing environment, and billing or invoicing for consumption of these resources. In one example, these resources may comprise application software licenses. Security provides identity verification for cloud consumers and tasks, as well as protection for data and other resources. User portal provides access to the cloud computing environment for consumers and system administrators. Service level management provides cloud computing resource allocation and management such that required service levels are met. Service Level Agreement (SLA) planning and fulfillment provide pre-arrangement for, and procurement of, cloud computing resources for which a future requirement is anticipated in accordance with an SLA.

Workloads layer 6066 provides examples of functionality for which the cloud computing environment may be utilized. Examples of workloads and functions which may be provided from this layer include: mapping and navigation; software development and lifecycle management; virtual classroom education delivery; data analytics processing; and transaction processing.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of one or more embodiments has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain various aspects and the practical application, and to enable others of ordinary skill in the art to understand various embodiments with various modifications as are suited to the particular use contemplated. 

What is claimed is:
 1. A method of accessing at least one memory location by one of a plurality of transactions in a multi-processor transactional execution environment, the plurality of transactions having assigned conflict priorities associated therewith, the method comprising: tracking execution progress of one transaction, the execution progress being a metric of work performed for the one transaction and comprising at least one of transaction instructions processed or cycles elapsed; receiving a protocol request packet from an other transaction for a memory location, the protocol request packet comprising an indication of execution progress of the other transaction; based on encountering a conflict between the one transaction and the other transaction: determining whether the one transaction or the other transaction has a higher assigned conflict priority; based on one of the one transaction or the other transaction having higher assigned conflict priority, continuing that transaction and refraining from a comparing of the execution progress of the one transaction and the execution progress of the other transaction, otherwise, comparing the execution progress of the one transaction and execution progress of the other transaction, using the indication of execution progress of the other transaction in the protocol request packet; and based on the comparing, deciding whether to continue the one transaction or the other transaction.
 2. The method of claim 1, wherein the deciding comprises: based on the execution progress of the one transaction being greater than the execution progress of the other transaction, continuing the one transaction; and based on the execution progress of the one transaction being less than the execution progress of the other transaction, aborting the one transaction.
 3. The method of claim 1, wherein the deciding further comprises: based on the execution progress of the one transaction being greater than the execution progress of the other transaction, sending a response to the other transaction rejecting access to the memory location; and based on the execution progress of the one transaction being less than the execution progress of the other transaction, sending a response to the other transaction comprising data corresponding to the memory location.
 4. The method of claim 1, wherein based on encountering the conflict, and prior to performing the comparing, the determining comprises: determining whether the one transaction or the other transaction is a super-transaction; and based on one of the transaction or the other transaction being the super-transaction, continuing the super-transaction and refraining from the comparing.
 5. The method of claim 1, wherein the metric of work performed for the one transaction is incremented based on instructions processed or cycles elapsed in a decode stream for the one transaction.
 6. The method of claim 1, wherein the metric of work performed comprises at least one of transaction instructions executed since a TBEGIN instruction, or cycles elapsed since the TBEGIN instruction. 